"You're not really black, you're white on the inside."

I thought this was just a South African thing, but someone linked me this and it seems to be a big part of the USA elections this year too.

https://www.youtube.com/watch?v=v56x9xsRJQI

Mmusi gets similar treatment here in SA for this election. Where does this idea that a smartly dressed black person with different views is somehow 'white' come from and why is it so prevalent, both here and in the USA?

Is it anti-intellectualism? Some form of tribalism gone mad? Or somehow acceptable treatment?