It might be in this case, but I would not object for a trans man to call themselves femboy if they look like a feminine boy, since for all that matters to me a trans man is a man.
You may not object but that’s not how the word is commonly used. Anyone can call themselves anything, but for words to have any meaning we need to have a gentleman’s agreement about when and how to use them, which is basically what a language is.
Anyway, I’m just explaining the word’s conventional meaning. I have no interest in arguing with you as if I were some kind of linguistic prescriptivist.