I live in NYC there are Africans of all types, trust me, they're not short 5'7 people. In fact they're on the taller side. Other than the pygmys, there aren't any African tribes that can be classified as shorter than the average American.
pretty sure that the average height in many African countries is, in fact, lower than the average height of white Americans in the USA.
I completely reject your assertions that all Africans are basically the same. You're flat out wrong.
If you want to say that African Americans are physically different from other Americans in terms of height (is there even data for this? haven't looked, but I don't know if that is true), please present some solid evidence for this... and consider other reasons than "Hey, Africans are all tall!" as an explanation.