Bastard, monkey, darkie, and many other derogatory terms were readily available around that time.
It's lazy writing and a favorite go to and likely a-historical considering it's Tarentino.

Also in the film one of the men is familiar with him, one is purporting to be a mexican, and one is purporting to be a british nobleman ... all of them use the term just as much if not more than the confederate in the flick.

I disagree with your statement.