If they were right, they might simply have other insights that mainstream medicine/society don't recognize- or the thoughts might be associated with metaphysics or other things the mainstream doesn't understand. Most people in psychosis, won't know they are in one, when the connection to reality is broken- although some, might. its a matter of degree.
I don't know if thought disorders make one able to see a past self, maybe they can. Many people without thought issues have the past life ability/belief system too.
A little off topic but consider this- shrinks and psychologists are considered be be the "professional" and proper authority on these issues, and in many ways they are.
If you see a past life, or have an unusual perspective, or hear hings/see things that others don't-and deemed too weird> they say psychosis.
But if you believe Jonah lived 3 days in the belly of a whale, or that literal resurrection of a clinical dead body happens, or hear a voice from the heavens- you are religious.