There are genetic differences between groups, but claim (a) is that there are such differences that contribute (substantively) to different outcomes -- as you wrote yourself. I don't know of any such evidence. It's controversial to say the least, but if you are engaging me in a discussion, you have the burden of proof on that point.

As mentioned, I don't really care for these types of discussions, because either way, it's not going to lead me to change my actions. My original motivation was to see what moldbug thinks of Jared Diamond's work (i.e. if he tries to refute it)

(a) and (b) aren't exclusive, but it could be that one contributes vastly more to the observed outcomes than the other. I happen to believe that this is the case with (b), as Jared Diamond explains. There is just much less evidence supporting (a) compared to that supporting (b).

I think you misread my last statement. Providing evidence for b means that you can't apply the argument that if b were false, then a. They are not exclusive, but at least one of them is necessary. I would assume (b) is false without evidence as well.