This is a potentially valid but minority interpretation of the statement "X is trans.", held by no trans people I have discussed the subject with (and it tends to come up, eventually, across a sample size of a dozen middling-to-close friends). The more common one, which all trans people I know hold, is "At some point during my development I gained a strong repulsion from the particular cultural bundle labeled with the gender which matches the dominant one for my sex." Generally this is also a strong attraction to the other major gender bundle, but sometimes the trans person finds that bundle equally off-putting and rejects both, and others exchange between the two regularly or hold themselves to be both. Also, the poor coding practices which evolution uses make this more, not less, likely to occur. Most people who study trans issues, history, etc. agree that there were probably high rates of 'masked' trans people in the past, who kept their personal identification preferences secret.

I doubt there's much neurobiology backing it up on a higher level than "Look! These brains are different! Let me make a huge list of rationalisations why." From a medical POV neurobiology could help, but isn't necessary to take action. ETA: Regarding the first point, finding neurological differences in different kinds of minds should be the default assumption, and shouldn't in general make the experiences of these minds more or less valid. I think validating or invalidating empirical psychology via biology with our crude tools is a fallacy, and wonder if there's a name for it. I suppose it could be some kind of a subtype of the naturalistic fallacy.