I've often thought the same, they are saying those who are evil apostates in their eyes have a mental illness. If that's true then we need help rather than shunning surely?
As for demons, yes when I was in that cult I, like many others, had depression and an elder and his wife came to see me. I was trying to explain how I was really trying to get myself well and I said I feel better when I get out of the house for a while. The woman immediately said oh I wonder if it's demonism if you feel better when you get outside of this old house (we lived in an apartment in a Georgian house). Not helpful! They're obsessed with demons.