YEC is not closely related to Flat earthers. I know many YEC that refute flat earth theory completely. The overlap of the two is small, but maybe there are a few outspoken ones on Reddit (it’s always the outspoken ones isn’t it?)
Just a hunch, but I think the correlation of flat earthers to schizophrenia is a thousand times stronger than any other.
They are more closely related than you think. Flat-earth is actually a natural conclusion from the assumption of Biblical inerrancy (e.g. 1Sm2:8, Mat4:8, Isa11:12).