That's a tiny minority of people and an ultra-specific belief.

I would say that the prevalence of the belief in fairy stories being real (aka Religions, Cults and so on) would be a pretty good indication of just how common and widely spread the Comprehension Handicapped are all over the World.

I think the modern flat earth idea started in the UK but I don't actually know of anyone who believes it, it's still very much a "village idiot" thing.