This is actually something covered in the F.A.Q. already under arrival! If asked, The Peacekeeper will gladly show anyone their world ending and how.

...characters will receive a harrowing vision of exactly what was promised: their world ending. While likely something canonical that could have destroyed the character's original world, it can also be something vague or unexpected. Plague, war, famine, etc. If they're CRAU and coming from another game setting, they will instead see that reality falling apart.

The Overseer's explanation isn't meant to refute this but rather reaffirm it. Character's worlds are rotting away i.e. ending unnaturally sooner than they should and what seems to be intentionally. She just doesn't know what is causing it.