At the risk of over explaining, this is the standard leftist, feminist, and Marxist folly of conflation. The song "Be a Man" is sung as far as the speaker is aware, to an army of men. In this context, very obviously, a "Man" is aspirational, exclusive, and performative, and necessarily not something any boy is born as. This is evidenced by the adult male listeners who do not meet any of the qualifications at first.

The song does not presume or insist on any physical attribute at all, because it is neither important nor distinctive between what a boy is and what a "Man" is. Instead it focuses on spirit, mind, and comportment. The twist is that a girl is listening, and she is indeed not a "Man" nor is she even a "Woman" she is just a girl. She embraces these attitude changes and rises into a positive example along with the now Men around her who have also been influenced by the instruction.

The intentional distinction in the original movie is that a "Man" is divorced from being a male. It is instead an ideal that even adult men do not become automatically.

The conflation now, is that being a good example of the male ideal, is the same as being a man. Plenty of women can learn from the male ideal and strive for it if it is appealing. Not only does nothing in our culture stand in your way; it has been explicitly encouraged for 50 years. The original Mulan was very nearly peak feminist messaging pre-Current Era. "Girls, be great examples of Men," was a very strong message all through the 80s and 90s, and while it was tedious, it was at least largely positive and focused on a pursuable ideal.

The conflation was inevitable, especially as we pivoted into the understanding that women can't actually BE men, so men must be demoted for the desired effect of "equality." Trying to be stoic, trying to be strong, trying to be self-sufficient, these were at least beneficial things to try, but the desired measured outcome wasn't materializing. The female domination in all pursuits didn't show up as believed, so men had to be destroyed instead.

There are too many ironies to admit in the new reality where girls try to physically transform themselves into boys while somehow missing that boys are hated universally by the same forces that are encouraging them to do this. They are being sent down a chute of pain with hell at the other end in every single form. Boys are being seduced similarly with the fantasy that they can escape the ire of the monolith by rendering their body unto Caesar to escape the societal lions. This is obvious entrapment, and pitiable for all caught in its lies.

Summary: Being a "Man" is not the same as being male. Self-evidently so from the material being referenced. Fucking communists.