What work is the word "really" actually doing here?
How about referring to the cluster structure of gender space. Of course, then we'd reach the conclusion that there are only two genders, and the traditional assignment is people to them is the correct one.
"Once gendered behavior has been determined, however that occurs, cisgender males don't say "I'm a boy!" for cognitive reasons that are substantially different from the reasons that transgender males say "I'm a boy!"
Except "cisgender" boys don't generally engage in questioning "am I really a boy".
As I understand it, there is a phenomenon among transgender people where no matter what they do they can't help but ask themselves the question, "Am I really an [insert self-reported gender category here]?"
The obvious answer is "No". In fact this experience seems suspiciously like trying to make oneself belief that one believes one's gender to be X.
...Humans universally make inferences about their typicality with respect to their self-reported gender. Check Google Scholar for 'self-perceived gender typicality' for further reading. So
Yes, people tend to vigorously contest things that threaten their identity. This tends to have no relation to the truth of those things.