100 is not a perfect score on an IQ test! 100 is the mean score. It's not a percentage.
I'm sure there are some genuine flat earthers out there, and I imagine that their IQs do average near 100 (perhaps a little lower). But I'm basing that on a general understanding of how people come to stupid beliefs, rather than from observations of individuals, because I'm not sure I've ever met a flat earther who actually believed what he was saying.