I suspect the reason Amber in the Shivering Isles is special is because it's sap from the Tree of Madness, which all the root channels come from.
You are assuming that mortals have a limited understanding and thus concluding that they must be wrong. This is not a very productive stance as you won't be able to argue anything at all. You can get allot more done, if you invest in some nuance and assume sources to be correct until shown otherwise. This is a reasonable assumption considering that this is a game and the narrators needs to get basic information across to tell the story.
There's a reason I assume that mortals wouldn't know; Red Mountain is the only volcano in Tamriel, and the only source of Ebony in Tamriel. And it's the resting place for Lorkhan's heart. It wouldn't be unreasonable for mortals to assume its presence is due to the Heart, instead of that it could just be a naturally occurring substance given the geological conditions. And it's certainly not a stretch to assume that they're not right about everything they say and write.
Daedric weapons are made from raw ebony which has been refined using the craft and magical substances of the lesser minions of Oblivion. The process is not a pleasant one for the Daedra involved, and the weapons retain echoes of preternaturally prolonged suffering endured during manufacture. Daedric weapons are the most rare and expensive weapons known in Tamriel. - Garothmuk gro-Muzgub, Smith - http://www.elderscrolls.com/games/morrowind_weapons_02.htm
We have a source to note that Daedric weapons are made from raw ebony.
Raw ebony is one of the most precious substances in the Empire, and most of the continent's deposits are here on Vvardenfell. Raw ebony itself is an extremely hard, durable, black glass-like substance, said to be the crystalized blood of the gods. Raw ebony is protected by Imperial law, and may not be mined or exported without an Imperial charter. Ebony smuggling is a profitable but dangerous source of illegal wealth on Vvardenfell. - Tendris Vedran, Alchemist
And we have a source to note that raw ebony is said to be the blood of the gods. Not unreasonable considering that the crash site of Lorkhans heart has the highest concentration of Ebony.
I'm well aware of the mortal perception of things and where it's derived from. However, there are a few problems with the mortal theory.
1) Ebony is one of the most highly regulated and rare substances in all of Tamriel. How does Mehrunes Dagon have so much ebony if he got it all from Tamriel?
2) Even if there was enough for Mehrunes Dagon's purposes, with the divine barriers in place, it is impossible for Mehrunes Dagon's underlings to be able to mine and then move more than a few pieces of ebony from Tamriel to the Deadlands. If Red Tower was actually active since the Heart fell into it, than it makes even less sense.
3) What do you think a mortal would classify Daedric equipment as? They've never been to the Deadlands, and there's only one volcano in Tamriel, so there's no way of them knowing whether or not ebony can form anywhere lava cools at a certain rate, or if it's actually the blood of Lorkhan. And it's not like we actually have any evidence for the casual belief that it's the blood of Lorkhan.