The best counter-argument is to point out the totally dishonest nature of the claim. If there's no such thing as white people then how come everyone knows exactly who these white people are when it comes to affirmative action, state-enforced discrimination, critical race theory, postcolonialism etc? The people who argue this stuff are not even remotely interested in a serious examination of race, they just use arguments of this type for the purpose of gaslighting. That's all this is. You can argue with rational and courteous individuals, but you would be an utter fool to argue with someone trying to gaslight you.