This argument has never made sense to me. EVEN IF there could be some kind of definitive way to show female vs male brains, and then some way to show that this group of males actually had female brains, that wouldn't prove to me that they were actually women, it would prove to me that their original thesis was false, and that actually some men do have what they were calling female brains after all. It's a miscategorization. Men and women are categories based on sexual reproductive systems.

If there is a significant brain difference in humans that is worth studying, and all women show it but the only men shown to have it are body-dysmorphic? or dress according to female stereotype? or have taken hormones? - or, whatever the scientific commonality of "being trans" is considered to be - then perhaps there is some knowledge to be gained about that part of the brain, but it won't change the nature of male and female.

Nonetheless at this point I am sure no TRA would actually want that level of scientific precision, since everything is by self-ID, so if a brain scan would be able to tell you "actually you are not trans" that would be unacceptable. So it feels very much like a have your cake & eat it too scenario