Who did Afrikaaners claim founded ancient African civilizations?
According to the first link that you attached here, the brief answer is that all of the white people who came to Africa (including the Afrikaners) believed that white people had founded the ancient African civilizations.
The idea here was that Africans were inferior. Because they were inferior, there was no way that they could have created any great civilizations. If the Africans did not create the civilizations, it made sense that white people must have because (as all the white people thought they knew) white people were simply better than everyone else.
This has been a pretty common idea in the minds of European scholars (until recent times). It was, for example, a major Nazi idea -- that all civilizations had been created by Aryans.
The whole phenomenon of apartheid and the notion of white superiority in South Africa stemmed from the fact that the Afrikaaners or "boers", as they are otherwise known, felt themselves to be the founders of South Africa.
For them, they were the civilized people who discovered South Africa and "tamed" the Zulu and other African tribes. Their victory in the Battle of Blood River simply reinforced this idea.