No need to question what is given. And no, the argument does not rest upon it. However, the notion that white people are superior to nonwhite people is definitely artificial. But that isn't the crucial issue here. What is is the issue of what we as a human society values. White supremacy is saying that certain human being are not to be valued because of false conceptions of those people. I equate falsehood with artificial, but that is secondary to the structure of misrecognition in white supremacy. That is the point--that structure is inherent in all ideologies that deny the moral equality of all people. Again, back to Hume, our sentiment for others is natural. It is an observation from nature.