> I don't know how far into this you want to go, but no, I don't think it's possible to think that there are biologically determined on-average cognitive ability differences between "whites" and "black people" without being a white supremacist.
You either are wrong or are speaking a language in which the phrase “white supremacist” means something different than it does in normal English usage.
> I recognize that there is a trope suggesting you're not a white supremacist unless you want there to be a biologically determined difference.
There might be, but I've never encountered it. And I'm starting to think you are not merely posting from a place using alternate English, but from a complete alternate reality.
Aside from you, I've never encountered anyone who seemed to think that the core of white supremacy was a belief in on-average differences.
White supremacy has two defining features (this is an elaboration of the two-point dictionary definition posted upthread), both of which are required:
(1) The belief that members of the White race, as such, are inherently superior—and this is reslly specifically about moral/value superiority—to those of other races. Universal or on-average biological differences may be pointed to as evidence of this superiority of value, but they are not the central point.
(2) The belief that as well as (and usually because of) point 1, above, members of the White race, as such, should be in a dominant role, and those of others limited to a subordinate role, independent of individual traits aside from race, in society.
Believing that whites have an on-average advantage in one dimension like IQ and neither assigning greater moral value to Whites as a class because of that nor believing that Whites should be favored in society does not make you a white supremacist. It doesn't even make you a pro-White racist, which you can be without being a White supremacists (holding belief 1 without belief 2 would make you a racist, but not a White supremacist.)
> The term means what it means.
Yes, and what the term “white supremacist” means is not “person who believes that whites have a higher average IQ than blacks” or “White Americans have a higher average IQ than African Americans”.
It's true that each of those (quite different, though often conflated) beliefs probably has some correlation with white supremacy, and indeed I'd be surprised to find a white supremacist who disagreed with either. But neither belief, alone, makes you a white supremacists or even a racist.
Just like believing men are on average taller than women doesn't make you a patriarchist, or even a sexist.