It's an interesting theory but I think you might be overanalysing it just a tad. I thought it was pretty funny and I don't consider myself a racist. As a matter of fact, my mom is from brazil and she's black.

I agree that it's not a particularly clever joke but sometimes a joke, or a pun if you will, is just so bad or corny that it becomes really funny again. I feel that's the case here. And then there's of course just the whole absurdity of the situation of Steven being quite racist while he himself is black.

Now I understand I might not fully grasp the connotation the word nigger has today in the us since I'm Dutch but I still think you have to see it the context of the time. The word nigger originated as a neutral term to refer to people of sub-saharan descent and back then was not necessarily a racial slur.

Django refers to himself as a nigger all the time and Dr Schultz Uses the world quite often aswell and you can hardly call him a racist, quite the opposite actually. Back then it was just a word that meant "black person". In holland we have the word neger which has the same origin as nigger and its not considered offensive. During the whole movie the word nigger is used in about the same way we still use the word "neger" in the dutch language today.

It all depends on the context in which it's said. Its like the the word jew. In itself cant be considered a slur but can easily be used as such by an anti Semite. Like saying to a jewish person "get off my property, jew! " or calling a non-jewish person a jew when they're acting greedy.