I think that Sargon was using the word in the context of the characteristics implied e.g. bad behavior, etc. He was essentially saying you are white people that are behaving poorly and doing the same things you say you don't like this other group doing so the name you give to that bad behavior is equally applicable to you. That is actually the only way it makes sense. If you look at that word as just a racial slur that includes everyone of a particular color, then calling the white supremacists that word doesn't make sense. He would just be saying you are white black people.
"There are things which, if done by the few, we should refuse to imitate; yet when the majority have begun to do them, we follow along - just as if anything were more honourable because it is more frequent!"
-Seneca
