We've all read that article on a trans person, a thousand frigging times, where the cissexist person goes back and forth with the pronouns, treating them as their CASAB before they transitioned and their identified sex now that that's generally acceptable to cis society. Everyone hates those articles, because they degender the fuck out of trans lives, trans people pre-transition, treat transition as a sex change when it's an assertion of sex, and ignore the fact that neurological sex is not plastic, no matter how much porn we may have seen or written that likes to imply otherwise.
But when trans people do the same, it's suddenly okay? It's their identity and we shoudn't question their identity, right? Bullshit.
The part of the brain that's sexually dimorphic is non-neuroplastic from infancy, so there's every reason to expect fluidity, non-binary identification, and discovering an identification that works better at a later date. There is, however, no demonstrable case of a boy-that-becomes-a-girl. There are, however, plenty of instances of girl-who-stops-pretending-she's-a-boy.
Trans people were never their CASAB, and a narrative chock full of internalized cissexism, a narrative that treats a young girl as a boy because she wasn't recognized as a girl, wasn't allowed to recognize herself as a girl, should be criticized (though perhaps more in abstract than on a case-by-case basis, because the best weapon against self-hate is acceptance). Narrative, especially one that relies on cis standards, isn't identity. Identity is identity.