It’s interesting.
On Woke Earth-2 men can become women, but a white person can never become a black person because of the different way they think about these identities. So, they think woman and man are social terms that refer to social roles enforced by oppressive systems. (Think of their phrase “Gender is a social construct.”) You and I think, and most people think, that woman = female and man = male which are terms of biology — that we are a sexually dimorphic, sexually reproducing species that is organized for reproduction in two ways, male and female, and that woman and man refer to this biological organization.
Wokes think of these identities as separate from biology and that woman and man are terms of social location, and that those identities are fluid. For them someone talking about biological women or men is a bit like someone talking about biological presidents or prime ministers — this is why they will say things like “people who menstruate” when talking about periods and women’s health, because to them men can have periods and vaginas because man is just a role that is divorced from biology.
All of this nonsense comes out of Queer Theory; its proponents and adherents think these identities are like identity prisons that need to be deconstructed, so that they can be so fluid and meaningless that anyone can be anything they want, no matter their biology. Trans is thought about as liberation from the identity prison of gender and oppressive systems. So trans is perfectly in line with how they think about those identities.
The way they think bout race is different. They think it is a social construct but that you can’t deconstruct race to make it fluid because racism is a regular, permanent part of life, so it is not possible to do that. They also think that skin color confers a knowledge of racial oppression and that access to knowledge makes it so whites for example can never know what it is to be black. So a white person can never be black because they would not have the access to a knowledge of racial oppression that a black person does. Instead what they want to do is accept the racial categories but do liberation politics with them.
Wrapping this up - they think you can destroy identity categories like man and woman, that this is freeing people from the constraints of the roles assigned to those identity categories, and that trans is a way to do that. So a man can become a women. However they think it’s not possible to destroy racial categories and that racial identity confers a knowledge of racial oppression that can’t be transferred to another racial identity category because of they way they think about racism, so a white person can never become black.
Yes, I know how crazy this all sounds, how incoherent it is, and how unhinged it makes me look typing it out, but that’s what they think.