She's been putting herself between the rest of the world and the Evenuris for a long time. The world was betrayed by the treachery and abuse of the Evanuris when they slaughtered her, their defender. There's no reason at all to doubt any of that. It's very clear, just as the image in the Fade of her literal murder.
The Evanuris have been gone for all recorded history, apart from the vague memories the Dalish bastardized into their culture. We have no idea what the Evanuris did apart from Solas (somewhat justifiably) calling them bad cookies. But remember - Solas has a vested interest in demonizing his enemies, and he outright says as much to the IB in their banters (about fighting Tal-Vasoth).
The Fade - we're repeatedly told not to trust what we see in it - is not exactly reliable when it comes to objective truth. We've read a lot of codex entries recently that cast Mythal - in comparison to her peers - in a more compansionate light (esp. re: the Titans). But that doesn't make her the hero in the story, and it doesn't make the Evanuris the villains who betrayed her.
The line doesn't make sense if we're just talking about a murder. It has to be something more fundamental. It doesn't have to be Solas creating the Veil - but it's something more than just pretend gods being dicks to each other.
If that were true, why not just kill him while he was weak?
Why do you think she wants to kill him, even if she's talking about him, which, remember, is not what I really think? I don't think she wants him to die. Wanting Solas not to commit a second genocide means stopping him, but that's not the same as wanting to gank him in an alley.
My point is just that we shouldn't take either her or Solas at face value, especially when they're just being so very forthcoming. Their whole schtick is saying just enough to push people's buttons to do their bidding.
She isn't necessarily referencing the same betrayal there, since obviously Flemeth's betrayal is unrelated, and she says "She was betrayed, as I was betrayed, as the world was betrayed!" So the "She was betrayed" part could be separate from the "world was betrayed" part. Though I personally think she is referring to the Evanuris both times.
But it's a running theme. It's obviously not the same act, but it's the same type of betrayal. Mythal, Flemeth and the world were betrayed by - thematically - the same thing. Honestly, the only reason I think it could be Solas is that I think it's a betrayal tied to "love". That's the story we hear about Flemeth - regardless of how it's glossed and who her lover is - and ultimately Solas claims he created the Veil out of love for Mythal (not the romantic kind, clearly).