A reasonable theory would be that the best, most productive areas were colonized first. The productivity / potential of those lands can have nothing to do with colonization and just be inherent to the land.
Did the wealth and GDP per capita in the Southern US increase or decrease after the abolition of slavery?
Righting a terrible, terrible moral wrong should come regardless of the short-term economic costs. To suggest that economic concerns trump overturning codified racism is one of the most disheartening things I've read on this site. (I do hope I've misread your comment as a support of apartheid.)
I did look up the data, and saw what you saw. However, his daughter comment links an article that claims "whilst there have been gains in some areas, overall, most Black South Africans are materially worse off now than they were under Apartheid," which I took to be the actual point he was making.