Ya, I can see that. You're point makes sense enough.

To answer your question: Honestly I'm distrustful of anything found within an in-game book. It could be 100% accurate, a mix of truths, fiction, and opinions, or an out-right lie (deliberate or not). Until I can gather enough evidence to prove for myself that any in-game book is one or the other, I'll remain skeptical of its information.

On this matter specifically:
On the one hand if the book is accurate, then they're not Volkihar; which will only corroborate several comments Harkon and Serana make to this point.
On the other hand if the book is not accurate, then they "may or may not" very well be Volkihar depending on the level of inaccuracy . The which doesn't prove that they are Volkihar. It only either puts the DLC writers/developers in a funny corner for not paying attention to their own lore, or still offers wiggle room to allow some corroboration of several comments made by Harkon and Serana.

At least that's how I see the matter.

The name of the castle at first blush does seem like a clever tell-tale sign that they're suppose to be the Volkihar. But the more I look into it, and the more I play the DLC content, the less I'm inclined to believe this is the case. Only the developers/writers can say for certain.

I will restate this just as someone else had, you can actually meet the vampire slayer from Immortal Blood in the game. It's a side quest. And, he is a vampire. So the book must have some truth to it.

If some random vampires took the castle from a bunch of mortals worshiping Mara, why would they call it Castle Volkihar?

I still support the idea that Bethesda left crap out to make it easier. Although they didn't need to make it so that phasing through ice would be a power since that could be difficult, they could have had an in game cinematic where the dawnguard get pulled under the ice. Issue here is, that would require several things. Making an ice path to the castle rather than taking a boat, and probably altering the storyline for the dawnguard side while keeping the vampire side the same. For the most part, dawnguard and vampire questlines are reflections of each other, so they wouldn't have done this.

So yeah, I say Beth just left it out to make their lives easier.

Actually someone in another thread said that this would be easy, you would have to give them the same spawn mechanic as Spriggans, replacing the trees with ice-sheets.

The thing is Bethesda was more worried about putting in boring, and generic dragons into the game, so vampires and werewolves got the short end of the stick, especially the vampires who were essentially the Oblivion vampires but with some slight changes.

Are the Glenmoril Wyrd actually vampires though? They must have enough on their plates what with their Hircine-lycanthropy business and sometimes being hagravens. Maybe the writer mixed them up with the Montalion or some other clan.

We don't actually know that they can't reach through the ice. We just haven't seen them do it (perhaps because of engine limitations).

fixed that for you.

I still wonder how the volkihar kept the order and other clans out of skyrim I still keep trying to think of what is the volkihar forte I mean other clans have immense strength(Quarra), crazy magical ability(Aundae), stealth (Berne), intellegence(Anthotis) and blend/politics(Order). All we got is that the volkihar are really cruel and paranoid but how does that keep the other clans out? Phasing through ice won't do much since they can simply stay away from the coast.

Nah, you are just obsessed with the Whet-fangs, they aren't as cool as the Glenmoril Wyrd.

Whet-fangs are ten times cooler than the Glenmoril Wyrd.

Not so much, what makes them cooler than the Glenmoril Wyrd? Plus the GW probably look somewhat similar to what your avatar looks like, where as the Whet-fang are probably Argonians.

Exactly. A clan of enlightened Argonian vampires that the Order borrowed their idea of using magicka to keep their prey in a coma-like state. Who knows what other tricks they have up their sleeve? The Glenmoril Wyrd, I haven't looked them up much and don't know what they do but I do like the idea of highly political breton vampires.

The idea of Argonian vampires is kind of cool, but we don't know much on the Glenmoril Wyrd except that they live within Breton cities, so I want to see how they do that, and what kind of abilities they have. I don't know the Whet-fang just don't appeal to me.

Wouldn't mind going to High Rock just to see them in action. And in all honesty I like the idea of a beast race vampire so the Whet-fangs interests me the most. Well at least until I find out what vampires haunt Elsweyr.

Yeah, I want High Rock because that is the perfect opportunity for the Order vs the GW, which would be cool in my opinion. But I would like to see more beast race vampires.

That is very true sir. Another good point.

Nah not really.
As I said, just the name of a castle is nowhere enough to take as even an implication.
Throughout history lands and holdings have changed owner with and without changing name, this is no indication.
The statue of Mara could have been there to defile in profane rituals, the fact is that we just do not know.
There is zero dialogue or anything else to say for certain one way or the other, except that we know they are not the original inhabitants and the castle is called Volikhar.

Given as how the Volikhar in lore are not described as a family of Molag-Bal worshiping truebloods, my money is on that they are something new alltogether.

Even if Immortal Blood's legitimacy is disputable, I don't think you can argue that the vampires we got in vanilla Skyrim or in Dawnguard are as original or interesting as the Volkihar in the book are, one being a copy/paste from Oblivion and the other being modeled very much off of Underworld (to put it in a polite way).

I agree with this, Bethesda could have done great things with Dawnguard, like the Volkihar vs. the Order vs. the Dawnguard, and they could have fixed the vanilla Skyrim vampires to be more like they are in Immortal Blood.

in a book they live under the ice, in the game they all live in caves or castles. It's kinda weird.

To dispute the whole "Paranoia" part really quickly.

As an outside observer of Dawnguard, I can't say I know all the specifics of the story, but I do know that regardless of you being a human or a vampire, Serana will spill her guts about who she is (her name, the fact that she's a vampire, the fact that she's the daughter of a very powerful vampire, etc), and ask you to take her back to their top secret lair. Harkon then immediately offers to make you one of the fold, rather than just trying to kill you as an outsider outright in order to preserve their secret.

Neither of these are stellar examples of key members of their clan being paranoid. Personally, I see it as quite the opposite: far too trusting of outsiders, the barrier notwithstanding.

Also, paranoia isn't simply living far away and/or not having contact with others. It's more about being anxious, cautious, or wary of others. I live in a fairly isolated area outside of town and don't go there unless I need something. That's not paranoia, it's wanting peace and quiet and disliking cities, yet it's pretty much the same qualifications you listed to support the paranoia.

Then you have the fact that they aren't located under frozen lakes, but rather in a castle. Not only that, but they aren't even in the same part of Skyrim the book suggests you'd find them (the book says they're in the east, Castle Volkihar is in the west).

Then you have the fact that their powers are radically different from what the book suggests.

So yes, I'd say the deviate quite a bit.

The only paranoia is there are a few constantly at each others throats believing in conspiracies that one or another is planning and believing everyone is trying to become lord of the castle. They tell you you cant trust each other and stuff.

Really I just think that Beth dropped the ball. Game companies don't typically add a bunch of mystery of this nature for the sake of never answering it.

They wouldn't release a DLC with vampires that you assume to be Volkihar that live in a castle named after them, just to not release a DLC saying they lied or we lead you on lol.

But the Dawnguard quest itself makes very little sense, so this not making sense is just another small thing in a long list. (I've given up trying to figure out who any of these people are and what they are trying to achieve).

1 person's opinion against hundreds. Bethesda should have done something with ice powers, and maybe even add a few under the ice hideouts.