I think there are a couple of misconceptions here. Firstly, how we divide people into Black or white doesn't correspond directly to their heritage, and it doesn't work the same for Blacks and whites. Almost all African Americans have a varying percentage of European ancestry, so if dilution of the African genome meant that people were no longer Black, there would be practically no Black people in the USA. (The only ones being recent African immigrants.) In reality, this is not at all how we talk about race. A person with as little as 25% African heritage in the West is still likely to be coded as lightskin Black, but in South African nations is most likely to be placed into a distinct third category, coloured.
This categorisation of course isn't an objective fact of nature but is arbitrary and based on historical white racism, which would emphasise the pure heritage of the white population while often paying little mind to that of Black people. When you realize that 'white ********' isn't really the erasure of all white genetic material or even our culture, but simply the destruction of this social construction of whiteness, it's easy to see why it is in face entirely compatible with progressive ideologies.
The second misconception is that the races as collectives are equally diluted by people mixing. This is only true if we assume equal rates of procreation within the races, something we know for a fact isn't true. If you look at it on a global scale, the average Black couple have more children than the average white couple. As the most extreme example, you could imagine a world where all white women either race mix or refuse to have children, while Black women still have Black children. Even if you go by the above definition where mixed people are not Black, in such a world there would be no white people born into the next generation, but Black people would still exist. Reality is of course different in that white children are still being born, but as white fertility rates are far lower than those of Black people, every act of race mixing will dilute the white race at a faster pace than the Black one. Then, if mixed people aren't Black enough for you, you can always imagine the next generation of Black men breeding our mixed *********, I'm sure you've seen the memes before:
View attachment 273993