I don't think I've ever seen anyone deny the plausibility of the brain being wired differently than the body. What I believe the poster is referring to, and which I've seen in the media many times, is denial that physiological sex-linked characteristics are fully expressed even if doesn't match the one the brain is wired for. If brain wiring can mismatch physiology, it demonstrably is not determinative of the biology the brain is attached to in any meaningful way.

I understand the motivation for this denialism: most social institutions that segregate by sex are motivated by the practical effects of physiological sex-linked characteristics, brain wiring isn't a relevant criterion for determining "sex" for these purposes. It is currently impossible for the physiology to match the brain wiring in such case as a matter of science. Since the social institutions around sex segregation are widely viewed to exist for good reason, it motivates denial that physiological sex-linked characteristics actually exist for people that want to be segregated according to their brain-wiring sex.