If you looked at, for example, the number of different countries/continents with significant representation in more than one large city... probably not a single country in Africa would make such a list. Possibly excepting South Africa, Egypt, or Morocco.
This doesn't speak for the actual methodology used, or this quiz, since ethnicity is more about culture than genetics (the same reason race and ethnicity are separate on the US Census - an Afrocarribean Puerto Rican is just as Latino as a white Argentinian), but whichever way you look at it, biologically or ethnically, it holds up. The only way it doesn't make sense is through the lens of the outdated concept of race. You're right, though - isolation between tribes is what makes this definition favor African countries in terms of diversity.