I think you're right that averages are not useful when scores are bimodal since that basically means there are two distinct populations being observed, and what is more important is what population you belong to.

I think the existence of this bimodal distribution is at least consistent with the "haters gaming the system" theory, though not conclusive.

The IMDB ratings for the last jedi do not seem to exhibit such a strong bimodal distribution: https://www.imdb.com/title/tt2527336/ratings?ref_=tt_ov_rt

Which I think leads more credence to the theory that haters are overrepresented in the rotten tomatoes numbers.