No the information is not false but what your pointing out are targets not seals. Targets can be activated by simply shooting them where as a seal ( to me ) implies that it can only be opened by doing certain things and / or by casting a spell at them. I've been where you linked to and there's really nothing special there, nor did I have to do anything special to gain access to anywhere there. I didn't see any seals of any kind, just the targets and a couple pressure plates. I agree with Nefarious One, it was just a little back story to flesh things out.

Personally, I think it was an oversight. I think the caches Florentus is actually referring to are the ones that can be opened with those Dawnguard Cache Keys. My guess is that the original intention for these caches was that they could only be opened by use of the Sun Fire spell. But somewhere along the way, the devs decided against that for whatever reason and neglected to edit Florentus's dialogue to reflect the change.

OK, I found an iron door that is down but I cannot find the chain or whatever there is to open it. Any hints? This is fun!!!

Yyyyeeaahhh that sounds like Beth, your probably right. But you never know, but I bet your right.

Are you at the top of the cavern bridges?

Or are you in the tunnel?

Edited to clear any confusion on what gate you may be facing.
Hope that helps

I am at the bridges....wait..oh I found an archery target and shot it! Thanks, guys! Lots of loot (treasure!) to be found!

Still weird that Florentius refers to an actual in-game spell, it seems quite a precise description for just a backstory...

What kind of loot did you find??

2xEbony ore, a bunch of Corundum veins, exploding ice, fire, shock bolts, other random stuff in a chest, lots of Blisterwort.

It's not so weird if those key-locked Dawnguard caches that actually do exist within the game were originally supposed to be opened with that spell during an earlier stage of development.

They can be activated by the spell he refers to, I have done it.

No they certainly cannot, as I am currently standing directly in front of a key-locked Dawnguard cache and have blasted it repeatedly with the Sun Fire spell to no avail. Are you still thinking of those archery targets in Dead Drop Falls? Because much like "archery target" is not the same thing as "sigil" it's also not the same thing as "key-locked cache." Didn't we already establish that you were talking about something else?

Just to be clear, the Dawnguard caches are those locked chests that share the same appearance as strong boxes and they're scattered in various places around the main map. It's easy to identify them as Dawnguard caches as they've been cleverly labeled "Dawnguard Cache." They can only be opened either with a Dawnguard Cache Key or by picking the lock---not the Sun Fire spell (although I believe this was the original intention before it was changed to a simple key-lock mechanism and someone forgot to update Florentius's dialogue).

Yeah I spent about an hour last night scouring both inside and outside of Fort Dawnguard to no avail. Went into both caves, including Dead Drop Mine, by the waterfalls in Dayspring Canyon, on the walls of the fort, and all over the inside. Unless I'm blind, I didn't see any sigils. Maybe a banner with the dawnguard sign on it can be burned revealing a hidden passage beyond?

More than likely though, it's just oversight. Shucks...