Racism is a system of economic discrimination against people of a particular ethnicity and the ideas enforcing that system. It's almost impossible for an ethnic minority to be racist unless they're mercenary.
Basically conservatives are saying "Would that the world treated everyone equally, but if you do anything to achieve that, you're a racist". Well, fuck you. Bet you didn't think of that, did you?
Actually, racism is the idea that certain races are somehow inherently superior or inferior to others. You see this in the stereotypes that all blacks are either athletic or lazy, that all Asians are good at math but terrible drivers, and yes, in the idea that it's impossible for minorities to be racist.
Now, racism in attitude has often led to economic racism, even today, but that doesn't mean "racism=inflicted economic disadvantage". Having spent time in prison, I can tell you categorically that stereotyping and assumptions of superiority or inferiority are FAR from limited to whites.
No, racism is basically structural. The ideas are very much secondary to the economics. How do I know? Racism is actually modern. It is not the case that racism is human nature. When, in around the 15th century, racism became economically vital for Europe (colonialism, ect) of course ideas arose to justify that.
When did I ever make the claim that racism was human nature? It's human nature to (generally) be most comfortable with those who are most obviously like ourselves, but "comfort" doesn't equal "superiority."
As for your economic argument, that's very much a chicken-and-egg proposition. After all, what defines a race? There have been times in the past when the English would describe the Irish in ways that wouldn't be at all out of place in a modern KKK rant about black people. Chattel slavery as we saw practiced by Europeans enslaving Africans is a relatively recent phenomenon in world history (at least, to my knowledge), but the idea that we should be wary and distrustful of people who are visibly unlike ourselves is FAR from new. Chinese and Japanese culture still have strong undercurrents of racism against anyone who isn't "one of us", and the caste system of India was originally set up partly along ethnic lines, with darker-skinned ethnic groups being relegated to lower castes. The Old Testament even gives instructions that the ancient Jews not intermarry with other tribes; at that time, ideas that we would label "racism" and "tribalism" were almost completely interchangeable, because, as you pointed out, the concept of "race" being defined by skin color wasn't around yet.
Even allowing, for the sake of the thought experiment, that your proposition may be true...how, exactly, does that make it "not-racism" for a black person to say "all white people are racist" or "white people in general are greedier than other races"...both things I've had said
to my face? How does it make their attitudes any less wrong? And if their attitudes AREN'T a form of racism, what would you call it?