I've certainly encountered TERFs who believe that brains are wired in radically different ways based on sex. Biological essentialism is used to denigrate people all the time, by TERFs and others. You may be right that some TERFs believe the opposite about the brain. But what TERFs all have in common is that they follow those beliefs with: therefore, trans people are invalid.

I think that brain science has no bearing on whether trans identities are valid. Trans people's decisions and identities are valid regardless of whether brains are inherently gendered or genderless.

I believe, based on the scientific and anthropological evidence, that the psychological gender differences attributed to biology are mostly (if not completely) shaped by culture. The existence of societies where gender roles are completely different from Western gender roles makes it difficult to refute that.