Also, if I "diminished...American white supremacist attitude" that's a good thing, right? They should be diminished.

They are trying to say that you are ignoring the serious issues that the US has with white supremacy. Your comment obviously isn’t taking away any power from the fucking Nazis running the US government, and no one is claiming otherwise. In this context “diminishing” means “disregarding, downplaying, ignoring”. They are saying that you are pretending that it is not a real and serious issue.

For example, in the US, our president regularly says wildly racist things. His supporters then diminish the incidents, saying that the President isn’t racist and we’re just overreacting.

The impact, diminished the impact of it. And much of slavery was basically "we had a war and instead of killing you, I'll allow you to live but you work for me, and maybe at some point you'll be able to buy your freedom", it wasn't all "Western powers kidnap countless innocent people and move them around the world so they can work and die as the inhuman beasts the Western man think they are". They're completely different things, and this attitude is at the core of the American soul, because it's at the core of the Anglo Saxon soul, because it's at the core of the Western soul (hence the resurgence of Nazi-adjecent movements and politicians in Europe nowadays, it can't be just shaken off/waited out, they need an entire ideological change) and it's the reason why Americans are so happy to bomb/okay with bombing innocents around the world.

This thing is not like other things, and maybe you're just ignorant of it, but you shouldn't be. It's basically just the result of Western imperialism on crack and it's not dead, the labels have just changed.