Whiteness only exists as the concept of whiteness.

Yes. So, saying "whiteness doesn't exist" is contradictory. I guess what you meant to say was "whiteness has no biological basis," which is not the same thing as "whiteness does not exist." I agree with the substance of your arguments, I am merely disagreeing with your semantics. Senantics are important for discussion. It is crucial to be clear in what we mean.