What is the evidence for it? I don't think we've been shown anything at all convincing.
- First, we have the Forbidden Ones.
"For abandoning the People in their time of greatest need, for casting aside form to flee to where the Earth could not reach, we declare Xebenkeck and others of her ilk exiled from the lands of the Evanuris. Beware! Their familiarity with shape allows them to travel paths unaided. They may be bound, but only the protection of your gods will fully shield you from their malice. They are Forbidden from the Earth that is our right."
From this... we know that:
1) "For abandoning the People" - The Forbidden Ones served the People.
2) "casting aside form" - That they had singular forms. It cannot be that they can assume a form because of the lines that follow.
3) "lands of the Evanuris" - why not say "lands of the People"? Or "Arlathen"? Seems a distinction between the elves and the Evanuris. Admittedly can be do to status.
4) "Earth that is our right" - the "Earth" is not the physical soil. Earth is not the name of the planet - so why capitalize it? Earth - I believe is the Titans and lyrium. I believe this means that the Evanuris and the Forbidden Ones both shared in the "right of the Earth" - why share with a group of four spirits? I believe it makes sense if the Evanuris and the Forbidden Ones are the same - meaning spirits.
Solas says he was Solas before he was Falon'Din. Did his parents name him Pride? If they new about Pride demons back then... it would be a really bad name to call your kid. The entire cosmology of Thedas is based off the Fade infecting belief.
BUT, if Solas was a Pride demon... it would make sense. His personality would make sense.
Also, Solas' personality. He is unimaginative. He is unbending. He has "hints" of feeling bad - but he is compelled to simply ignore them to carry out the SAME thing he's been doing for thousands of years. Presumably... fighting the Evanuris. It took him thousands of years to formulate a plan to change the world.
It's funny that he "claims" he's been sleeping in the Fade for THOUSANDS of years... or, maybe he just went home and dwelt in the Fade. And why did his power diminish? We know a spirits power can diminish. We've yet to see a mage lose power outside of Tranquility.
Flemythal - also hasn't changed her song for thousands of years. These creatures are incapable of an original idea. They're either the worst written static characters in the history of literature... or, there's a reason for it.
The very idea the Mythal can possess Flemeth says absolutely that the Evanuris were spirits. We've seen this with Vengeance... and Wynne's faith spirit.
I don't know about "the People" - I believe they were mortals who were enslaved by spirits (the Evanuris, Forgotten Ones and Forbidden Ones).