It would be worth mentioning in this argument that Holocaust denialism reflects having an agenda of rehabilitating the image of the Nazis, just like frivolously asserting the presence of a genocide (which white supremacists frequently do, as you allude to with the South Africa example) serves its own agenda, frequently an exterminationist one. See: https://en.wikipedia.org/wiki/Accusation_in_a_mirror

I don’t think the Xinjiang genocide myth is being used for such a plainly exterminationist purpose like, for example, Israeli or Afrikaner claims about genocide are, but it still serves as an inflammatory causus belli against an other.