It's a joke.
Some of us noticed that in the UK they distinguish between "White African" Britons and "Black African" Britons (the "Britons" part is sometimes left off, a bit racist imo). It's quite valid there, as they have a lot of white immigrants from South Africa and from Zimbabwe. The US however doesn't make that distinction, maybe for racist reasons or maybe because there aren't enough white immigrants from Africa to make a fuss about it. Lacking the demonym "White African-Americans" those people can technically be called African-Americans. I don't like the term myself, I think the US should use descriptive terms like "black" for race, instead of harking back many generations for where everyone "came from". But I'll use it if African-Americans choose the term.
Jokes are never so funny if they have to be explained, are they?