To be fair, that's what the existing Dalish believe - and given that it's been proven time in and time out that their lore is piecemeal at best I really can't put much stock into their views in regards to the actual truth. If you've ever played telephone, you know how hard it is to keep information straight - even if it's only a few minutes. Now add in over a thousand years, slavery, destruction of their entire culture not once but twice, the rest of the survivors now scattered to the four winds or in alienages, and you get one hell of a mess. In the Dalish Origin for Origins they didn't even know that Dwarves apparently traded with elves, and that temple in the Brecellian forest played host to all races which everyone found surprising as well.
On the other hand they don't have the time to gather knowledge as one might think. Most of the time they have to run from human groups and villages because they are constantly hunted, they probably have a bit of actual data (books, writings and what not) about the old elves but makeup stories of their own to satisfy a need to know their own history. Even if you knew the truth about facial tattoos in your fallen culture, would you rather live an ignorant lie or tell everyone the ugly truth?
And the Humans know of the Dalish culture because the conqueror usually preserves some information from those they conquered if only to build upon existing data. Solas may have killed her, but the possibility exist that she may have taken control of Solas as well doesn't it? It wouldn't be the first time that she used her magic to control another.
All I am saying is the Dalish have been hilariously wrong about their own history, but we don't know that those who founded the Dalish after the fall of the Dales actually sought to preserve all that they had learned about Arlathan, or if they only kept the "happy stories" or made stories of their own. When I look at the Dalish now, I see them as naive children when compared to the Elves of the Dales, or Arlathan. They don't mean to be wrong, they just happen to be. But like the OP said it is to be expected after years subjugation, cultural genocide, infighting and time.