Yes, I think this is a possible way to go. Tell her you don't know what 'transmasculine' means, and you really don't understand what she means when she says she feels happier 'as a boy'.
What does she mean by 'as a boy'? ... "No, I'm sorry, I just don't get it. What is it you feel happier as? What does 'being a boy' mean?"
"You're a girl. Are you?" "Oh, well, but what does "girl" mean?" "And what does "boy" mean?"
"No, sorry, still don't get it."
Keep asking. Try to appear sympathetic. But, really you don't understand what she's saying. Let her try (and fail) to explain it.
Good luck.
I can’t help but picture you handing the OP an electric cattle prod as you calmly say ‘Good luck’…
Gosh, what a horrible way to address it. Please don’t talk to your daughter this way, feigning understanding whilst trying to trip her up. That sounds so manipulative and conceited - and also really creepy and unnerving. I get a sort of ‘Professor Umbridge’ vibe from this - all smiles while having a deceitful motive. If you don’t agree with what she is doing and you believe she is basing this on misconceptions regarding gender stereotypes just be upfront about it and explain it properly yourself.
Who would seriously treat their child this way?? Fucking crazy cakes, you’d make a great villain in a torture film.
‘No I don’t understand…’ big ominous grin - bzzzzzzz electric shock. ‘Let’s try this again…’