In a child comment you make the case that racism is when some group holds "tribal/identitarian" notions about an "other" group and attributes essential characteristics to them. You say this has existed throughout history and did not begin in modernity.
Ok.
That doesn't actually disagree with the "20th-century theory goo" though. The goo says that colonizing European empires created "new discourses" about race that served an ideological function in their empire-building. That's a far cry from "those people over the mountains there are barbarous."
The notions of "whiteness" and "blackness," as they still function today (against increasingly strong currents of critique) are clearly something different from that. In pre-modern times, a Viking and a Frank wouldn't have seen themselves as having anything in common. But now (anachronistically) they're commonly "white."
It seems like the confusion here--and I see it repeated all over the place--comes from a failure to clarify terms like "race" and "racism."
There's "racism" in the sense of a prejudice against some other-group that we can identify because they look or act different, and they live over there and we live over here. That has probably been around as long as different kinds of people have been encountering each other.
And then there's "racism," the ideological function where European empires constructed whole new ideas about groups of people that previously would not have seen themselves as having anything in common, in the service of expansion, extraction, domination, slavery, etc.
If you think that's all goo, that's fine, but you need to provide a stronger counter-argument. None of the purveyors of the 20th-century goo actually believe that no one hated other-groups before modernity.