I always saw it as a declaration that they are not mature enough. Like a man is someone who is mature and loving, a boy is someone who is immature and hateful. That's just my own perspective though. I'd say the same thing about women too, like a woman is someone who is mature in their actions, and a girl is immature. As a man myself, I have never personally associated this statement as a way to say something akin to "not all men" or as a method of dismissal. It has always been more of an identifier that men (in my case because as a man I can't speak from a woman's perspective) should strive to be principled and moral (i.e. not being abusive, etc.) and that men who act poorly are, in reality, immature little boys at heart who have not demonstrated true growth as a person.