The lack of strong African players is due to lack of infrastructure, and lack of top class competition.
It is noticible that Egypt places very high on the African list, I suspect that is because of proximity to Europe.
South Africa has a horrible track record, we've had this strange policy where almost all funds and resources are pumped int junior chess, and as soon as players reach senior level there is nothing left for them to do.
For at least 15 years, more than 90% of registered players here have been juniors.
That clearly illustrates that junior players do not translate into senior players. There are only a handful of SA players with a FIDE rating, tournaments are too far and expensive to get to.
BTW, I once played 5 blitz games against Amon Simuntowe, in a Guinness record attempt for the most Blitz games over a 48 hour period. I don't think the 5 games had a total of 70 moves, he absolutely crushed me after having been awake about 36 hours, lol
You said there are unknown black chess players that can beat top GMs, which is a completely outrageous claim. Why would you say such a thing?
What if we were saying this in the 1970's: "There are unknown black long distance runners who can beat top Olympians."