I would like to point out that at this point, the only information we have about pre-Fall Elvhenan comes mostly, if not 100% from Solas and Abelas has like two vague lines. I'm not entirely sure I'd be all, "The Dalish are stupid, lol, got everything wrong, lol," because as of Tresspasser some parts of their history, like the Dread Wold defeating their Gods, sealing them away, and taking their Empire/Immortality from them are accurate. Also, outside the elitist perspective of the Ancient Master Race Hermit Elves, the Tevinter Imperium did destroy the Elven Empire, as any information left was utterly whiped out via a massive Blood Ritual sinking the remains of what might have been Arlathan or some other important Elvhen city.
Also, Elvenhan was allegedly founded about 8000 years ago years ago, and 4000 ish years ago the Elven Empire fell. In our world, 3000 years ago was the hight of the Egyptian Empire, and really we know little about that, only fragments, and there was no world shattering magical genocide that happened to the Egyptian Empire.
Credit where credit is due. The Dalish know a lot for a society of nomads who wander the forest, hiding from humans, and searching for magical artifacts that were part of a system of physics (Fade/Thedas side by side) that no longer exist. It'd be like trying to put together a 4 dimensional puzzle in our 3 dimension world. Impossible.