The fact that Hitler was morally depraved explains why initiated the holocaust, why he invaded Poland, and so on.

If a "moral fact" is a "fact" which relates to a "claim about morality" like "thou shalt not threaten visiting lecturers with pokers", then how is this a moral fact? Unless the moral fact is "people named Hitler should commit genocide", in which case Hitler was never morally depraved at all. But "Hitler was morally depraved" is a fact about Hitler, it says Hitler's beliefs are different from "moral", where "moral" might be a selection of "moral facts" or it might be, say, my beliefs, or yours.

A geologist observes a volcanic eruption. We can explain this volcanic eruption by referring to pockets of magma beneath the Earth’s crust, high pressure cavities within volcanos, and other geological facts. However, it seems possible that we could also explain volcanic eruptions purely in terms of fundamental particles. It would be quite a lengthy explanation, no doubt, but so would an entirely psychological explanation of our moral observations. By Harman’s lights, then, we should not have any ontological commitments to magma, volcanic cones, and whatever else geologists use to explain volcanic eruptions. In fact we shouldn’t have any ontological commitments in the so-called special sciences, even Harman’s psychological facts must go.

This isn't begging the question so much as it's a red herring. It's true and widely accepted that the notation of stress tensors and weird made-up crap like a "mean-free-path" is a far more useful description of geological mechanics than trying to wrangle a Feynman diagram with (seriously) 1000000000000000000000000000000 sinks and sources. But that doesn't by itself demonstrate that the language of moral facts has provided us anything at all.

Yet this does not quite reach the true objection, which is this: we expect that a moral realism refers to the morality, not a morality. But it is widely accepted that the various "effective" theories of physics, chemistry, et cetera, come with a great deal of alternatives which may even make different predictions in certain situations, and we generally have to switch between models and theories to handle, say, the dynamics of magma versus the dynamics of the ocean, or of the atmosphere, or a fusion plasma, or an LCD display. It would seem to violate the central goal of moral realism to argue that different moral theories should be applied like this to different social situations; in fact, that position is usually called moral relativism!