I think this is a really interesting observation.But I don’t think it’s fair to say that average utilitarianism “avoids the repugnant conclusion”.If the world contains only a million individuals whose lives are worse than not existing (-100 utils each), and you are considering between two options: (i) creating a million new individuals who are very happy (50 utils each) or (ii) creating N new individuals whose lives are barely worth living (x utils each), then for any x, however small, there is some N where (ii) is preferred, even under average utilitarianism.There are many serious problems with average utilitarianism, not least that it doesn’t remove the repugnant conclusion anyway . So although I think this refutation of solipsistic swamping makes sense and is interesting, I don’t think it increases my credence in average utilitarianism very much.
Indeed, whether AU avoids the RC in practice depends on your beliefs about the average welfare in the universe. In fact, average utilitarianism reduces to critical-level utilitarianism with the critical level being the average utility, in a large enough world (in uncertainty-free cases).
Personally, I find the worst part of AU to be the possibility that, if the average welfare is already negative, adding bad lives to the world can make things better, and this is what rules it out for me.
I think you are correct, that there are RC-like problems that AU faces (like the ones you describe), but the original RC (For any population, leading happy lives, there is a bigger population leading nearly worth living lives, whose existence would be better) can be refuted.