You made a similar post the other day so my answer is the same: 'purity' is not a pre requisite for racial or ethnic advocacy. I, frankly, don't even know what it means and have never used it as an argument for why white people should collectively advocate for their interests. I only ever see it used as a sort of slur by the left who misunderstand our arguments and misrepresent them.

I've also genuinely never heard anybody use this argument against non-white groups which is why for me it's such a suspect form of argumentation.