Just get the Afrikaaners to conquer everyone else before the British come do that when they do the SA blacks speak Boer and have boer culture. 
I can believe that some of them have small amounts of non-European ancestry but I am skeptical that most do. It seems to be a popular myth for those with long roots in a settler society (whether it's the USA, Canada, Australia etc.) to claim that they have some "native blood" but DNA studies actually usually disprove this. Those of actual mixed ancestry frequently formed parallel societies, like the Métis or Coloureds.
There has been no study of Afrikaners DNA unless its new to me, however Apricity (international white nationalism website) has last I checked about 100 DNA profiles and all of them have non-european ancestry.
If I remember correctly since it has been a couple years the owner of the website has Angolan and Malaysian ancestry which changed his tune towards mixed race people.
But basically yes 23andme has shown this common thread.
Exactly, those are not myths.
Similarly, A recent study has shown that more than 10% of whites of Louisiana and South Carolina have some African blood and most French Canadian have some Indian blood. Miscigenation is in every colonial society.
I'm not saying that there are zero white people in these countries that have indigenous blood, just that it's exaggerated in a lot of people's minds.
The case of the American South is interesting: it seems that both white and black people claim "Indian blood" as a way of denying their ancestry from the other group.
Well, people claimed otherwise throughout most of our history and even today in some circles. However, the articles mentioned here are pretty straightforward IMHO, why do you particularly think that there's an exaggeration?
SighAmong anglophone settlers, in particular, there is a marked tendency to exaggerate or simply invent indigenous ancestry. I don't have a link offhand but I'll see if I can find one.
Even in the case of French Canadians, as your link notes, the genetic contribution of Amerindians is absolutely tiny, less than 1% of most people's genetic heritage. That's not indicative of widespread racial mixing. Rather, as the founding population was so small, there may have only been a handful of intermarriages at the beginning to create this genetic contribution.
The truth is uglier than most imagine: Interracial relationships in European colonies were relatively accepted in sparsely white populated colonies (and in the 18th century many were). But only white male-native female. And as soon as there was an influx of white women, the general attitude changed. White women needed to be protected against the wild lust of the indigenous males. White settlers therefore started to live in separated areas. And the numbers of interracial relationships declined. I have read independent studies, all describing this phenomenon hapening in 19th century colonies like South-Afrika, India, Congo and Dutch India.
The truth is uglier than most imagine: Interracial relationships in European colonies were relatively accepted in sparsely white populated colonies (and in the 18th century many were). But only white male-native female. And as soon as there was an influx of white women, the general attitude changed. White women needed to be protected against the wild lust of the indigenous males. White settlers therefore started to live in separated areas. And the numbers of interracial relationships declined. I have read independent studies, all describing this phenomenon hapening in 19th century colonies like South-Afrika, India, Congo and Dutch India.