It is an interesting quote, because we know exactly how Flemeth was betrayed: she "sold" herself to a powerful lord that desired her so that the man she loved could have money and exit poverty, but the lord was jealous and killed her lover instead of giving him the money. The betrayal was not keeping with the deal.
The Dalish legends and beliefs has many example of deals like these, everything comes at a cost and someone who refuse to pay his dude is doing something very bad.
"The world was betrayed" is easy to see under that optic: Solas broke the deal (balance) between the Fade and the "earth" when he put up the Veil.
That is also why I don't believe Mythal want to avenge her death, that is not what was broken, something else happened before she was killed, someone broke a deal made with her. In fact, her death might have been a preemptive strike by whoever broke the deal.
She was murdered by the Creators...I suspect she didn't have good relationship with them anyway.
But I can imagine Elgar'nan betraying her , dude had a bad temper and she was keeping him (and the rest) on a leash, but I doubt the one who defeated the Sun was happy about it.
What I think happened is the elves caused or found the Blight , yes it was bad but a really powerful tool granting "effective" immortality and you can use it to mind control everyone affected.
The Evanuris could be destroyed or at least badly damaged , Mythal ending up a whisp , a shadow of herself.
With the unrest caused by the Forgotten Ones , and Fen Harel all of those not believing in the Godhood of the Evanuris , I think the elven Gods started sweating big time.
Sooner or later , the People would know they weren't divine and could fall just like the rest .
We also know Mythal was using a form of mind control or binding at the very least with her Well , although it seems no one was forced to drink from it.
So yeah I think the Evanuris knew about the Blight and its power and wanted to truly "ascend ", I suppose Mythal opposed this and well bam ...
I aslo assume the rest of the Evanuris kept on with their plans of tainting everyone and themselves and that's why Solas had to seal them away in a hurry.