“Before whites, no one lived in South Africa. It was only after white settlement that tribesmen migrated from the north to South Africa for jobs.”
You got that nonsense from where?
While true enough the African tribes in South Africa had less productive abilities and organizations than those from Europe who built colonies there, it is nonsense to pretend they were not even there until after the colonists arrived. No unbiased and independent history of the region would claim that.
The allegation is that the population was scanty. And that might actually be true. South Africa is about as far south of the equator as El Paso is north — a subtropical clime. The native population preferred more tropical climes.