you would need to define 1) "sex", 2) "gender", 3) "gender identity" and the differences between the 3 of them.
also, it is true that "male" and "female" brain doesn't necessarily encompass all kind of brains "types" out there (which i'm guessing is the basis of their argument) -- which is why trans women (or sometimes, gay) have brain similar to natural women's brain, and trans men (or sometimes, lesbian) have brain similar to natural men's brain. following that logic, even though they don't identify with their birth sex, they still fall into the 2 "genders")
however, i have yet see an article that try to categorize or study "non-binary" people's brain to prove that there are explicit differences between their brains vs. the 2-gender brains. so right now to me the "not all brains can be categorized into 2 boxes" is still a theory (but, they may have read something, so I'd ask for studies or articles)
