Africa's higher human diversity is mainly in low-population groups like the Khoisan. The big groups are not nearly as diverse (although they do carry some Khoisan genes, for example).
Indeed really not true. For example: I am quite mixed, declared my own race on an unclaimed spot of non-scientific identity-formation, and despite what some would say and discounting any possible future irradiation, my genitic diversity is absolutely zero ;-)
It's like we've split the encyclopedia into three books: A-S, T, and U-Z.