Well it is logic, strengthened with a little diamond.
I don't think the Bantu Cows(and other animals) reached the Bantu in South Africa before european contact, plus they didn't have as much time to compete with the Europeans.
The Bantu got to South Africa around 900 AD, by that time Europe was already fighting each other under feudal states.
What surprises me more is the Swahili, they were already in small states competing with each other like the Maya and Ancient Greeks, plus they could've traded for technology with the Arabs long before Europeans came. Not to mention the effect religion(islam) had on the area.