Because you said "it seems" which suggests you are guessing but don't actually know.
And also because you think your own biased view of what is "very girly" has something to do with your child's gender identity.
So maybe you have talked to them about it, I don't know, but it doesn't sound like you listened.
I mean this alone-
Even if it breaks my old person brain a little.
It should no more break your old person brain any more than it should finding out your child is not heterosexual. If it is breaking your brain, you need to check your prejudices.
I'm glad your wouldn't do anything about a child's self-defined gender, but you still need to look at what you're saying and how you're judging.