It depends what you mean by 'supremacy', really; I think that's a hurdle many people trip up on (and it's probably been made intentionally ambiguous). Do I believe that white people are 'supreme' in terms of their IQ or physical ability or general human worth over other races? No, I don't. Do I believe that white people should be 'supreme' in countries which they built (as in they should maintain a supermajority of the population, and they should dictate the national character)? Yes, absolutely. I believe every race deserves that 'supremacy'.

I don't like to get into these IQ/physical ability/aesthetics contests with other races. I don't really care where white people rank on the totem pole. They're my people, and I would want to preserve them whether they were the dumbest or the most intelligent, as I would expect from any other race.

Yes, they exist, in the same way that there probably really were people practicing witchcraft in early modern Europe. The existence of witches doesn't mean they were nearly as prevalent or as powerful as the witchfinders claimed, nor does it mean that the witchfinders could identify witches with any accuracy.