I don't think that's what the GP was going for... rather, implying that flat-eartherness is uncorrelated to IQ and, thus, the average IQ of flat-earthers is the same as that of the general population.
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.