She in fact never said it herself.
I quoted the entire section you are referencing.
And my point this entire time has been that Thalyssra ineptly bought those lies without verifying anything for herself.
We do know that. Chronicle covered how Suramar expected the Legion to win and burn down the world and yet thought their shield would keep them safe and leave everyone else for dead.
Sounds like headcanon.
On this point I am not. I am arguing against a tangent premise you brought up: