Bullying is always wrong and must be addressed, whatever the supposed reason for it in some children's minds might be.
I always hated dresses as a child, always wore trousers, loved sports, had many male friends and played all sorts of games now coded as male. None of that made me a boy, only a girl who wanted less suffocatingly rigid gender boxes.
I thought feminism achieved that? But at some point the Western world decided to retreat from that (most of the rest of the world never visited the feminist ideas to begin with) and to welcome sexist stereotypes and 1950s gender roles for women and girls back. But now they are supported by both the far right and the far left.
If not wanting to wear dresses or being good at sports makes a child a boy, how does that really differ from the old sexist view that boys should not wear dresses and that boys should be good at sports, while girls should wear dresses and probably should not care for sports?
We end up with the same rules in either case, only difference being that the sexists base the rule on innate views about sex while the genderists base it on innate views about gender.
Neither of these allows much scope for feminism.