The brain-sex thing is them conflating anatomy and neurology, and calling that proof of gender because outliers exist.

Like, male brains are typically larger than females, and female brains typically have a larger hippocampus than males. Male brains typically have more gray matter, and female brains typically have more white matter. Men are typically better at visualization and spacial skills, and women are typically better at fine motor skills and details. Men and women experience emotions from the same part of the brain, but one of us typically experience it from things firing off on the left side and the other of us typically experience it from things firing off on the right side.

Key word here being "typically".

So then along come some scientists wanting to examine the brains of trans people. They find some anatomical commonalities between the brain of say, the trans women studied and real women. This now proves to TRAs that trans women are women biologically.

Nevermind that anatomy/biology is not cookie-cutter, brains aren't molded identically on an assembly line, and there are things that can literally change/influence your brain structure over time ... do not look any farther than the statement they desperately want to believe is true.

It's the same thing they do with intersex, and two-spirit, etc. They say, "see, these outliers exist that don't fit YOUR mold, so that means MY theory of fitting your mold is actually correct."

It's deliberate misinterpretation, and ignoring actual facts in favor of a narrative.