It uses a stereotype – “black men are called winston”
That’s not really what it’s saying though is it, the stereotype here is more “people called Winston are black.”
I don’t really hold with the Daily Mail attitude that you’re automatically being x-ist as soon as you point out any difference between two demographics. I don’t really think we need to be reaching for the pitchforks for a joke that hinges on someone in history having an unusual name. Is it any different to saying “Why was Bruce Lee famous? Because he was the last non-Australian to be called Bruce”?
Incidentally, racist or not, I’m struggling to think of a famous white Winston other than Churchill, so it’s not wholly outrageous. The only other Winstons that leap to mind are Zedemore in Ghostbusters and a Lenny Henry character.