No, what a twisted interpretation. 

That "black junior fellow" was neither belittled, called a slave, squished, or shamed. How is acknowledgement of a past injustice shaming or belittling? On the contrary, if anything  was belittled, by making fun of it, by pointing out how anachronistic it sounds today, it was the grand title of "Master".

It is the inappropriateness of the whole concept of master and slave today that was being pointed out in the joke, lame as it was.

What is amazing is that anyone could actually think he meant to belittle, shame, etc. , as if having once been oppressed was a source of shame today.