That's an interesting idea, but it could be a bit of a stretch. Dutch Boers, later called Afrikaners tended to aspire to be farmers, rather than merchants. In the last 65 years or so, Afrikaners did move into the big business world, but that was a nationalistic reaction to the economic dominance of English-speaking whites.

If I had to hazard a guess, I'd say that the rise of the CA as CEO has to do with the complex conglomerates that were created during the apartheid era. Companies had limited expansion opportunities, so they'd use their capital in unrelated businesses. South African Breweries, for example owned a large supermarket chain. The focus on the bottom line in these complex conglomerates rather than a specific business was presumably a great fit for bean counters. And even though the era of great unfocused conglomerates has ended as expansion opportunities opened up when apartheid ended, CAs were deeply entrenched in the corporate world, and the phenomenon of CAs on top has continued.