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 actually a remarkably good one. If we continue to mix for several hundred years more then it won't be. For now, it is.