“We find nothing beyond traces of sub-Saharan genes in areas where Roman legions were located”
And that is all you would expect to find or need to find to demonstrate that there was some diversity in the Roman Empire, including what we would now call racial. What percent of Iraqis have Zanj ancestry? How many Portuguese have descent from the West Africans who comprised 10% of 15th century Lisbon? Traces. In my own family I have seen descendants of my purely Bantu great-grandfather with blonde hair, hazel eyes, and milky white skin. Kids who you could not convince anyone were of near black ancestry.
People do locate the past in present terms, look at Medieval Christian iconography, and no doubt today’s norms “color” the past, so its unsurprising anti-racists take this as far as past racists.
Check out Frank Snowden, Jr.’s _Blacks in Antiquity_ for an informed and largely unromantic discussion of this specific topic.