And it gets some weird amount of focus seemingly purely due to modern sensibilities in an anti racism drive and to create a sense of belonging.
What does it say about people who relentlessly push back on it? It's not like people are arguing Rome had the same make up as the US. The thesis is fairly weak ("a small trickle of black people shows up because Rome was diverse and a unified empire that favors large scale population movements") and yet a large contingent of people seem intent on saying "no, not even that". I didn't mention anything about anti-racism drive, that's odd of you to bring it up.
They didn't just go back, didn't get selected out so that they didn't routinely show up in genepool research. They just aren't there in the demographic data.
https://www.youtube.com/watch?v=dnfBYO53jhg
(Africans in Anglo-Saxon England)