I think this is one of the things that really worries me about the whole "TWAW" thing. I have a friend who has a trans child (FtM, with no medical intervention so far, so basically short hair, boys clothes, and uses he/him) who is firmly in that camp; the child was astounded when he got his period because "boys don't have periods". Then read somewhere that "boys can have periods too", so decided it was ok.
This child then downloaded some fitness app which gives you tailored workouts. Of course, selected "Male" for gender, and was given workouts suited to teenage boys and men. Then pushed and pushed to try to achieve the goals, and came very close to serious injury trying to lift weights that were far too heavy because "the Internet says boys should be able to do this".
If that child grows up genuinely believing that he is Male, what happens in medical scenarios? He needs to understand that he can dress as he wants, use the name and pronouns he wants, but he is still in a female body and needs to access the correct medical care for that body, otherwise serious harm could follow.