I don't know where this concept of China exterminating all Africans
came from, but there isn't a snowflake's chance in hell of anything
even remotely like that coming close to happening. It does sound like
a racially-based science fiction fantasy.
Most people think of Africa by what they see from the projections that
appear in ordinary maps. A lot of people believe that Africa is about
the same size as Texas.
So I like to point out that Africa is about the same size as:
the United States PLUS Alaska PLUS Europe PLUS China,
and there's still a little room left over for New Zealand.
So perhaps a more interesting science fiction fantasy story would be
about the Africans exterminating China, Europe, and the United States.