I think you are overreacting a little bit.
(Standard European) Dutch was the official language of the Boer republics of Orange and Transvaal (officially the Orange Free State and the South African Republic). Likewise, the South Africa Act 1910, i.e. the original monarchical constitution of the Union of South Africa passed by the UK Parliament, declared that English and, again, Dutch were the official languages of the Union. Afrikaans became an official language pretty much through the backdoor when, in 1925, the Parliament of the Union passed a law saying that the references to Dutch in the constitution should be interpreted as "including Afrikaans".
Yes, Afrikaans and Dutch are now separate languages (in terms of phonology, grammar, vocabulary, and spelling), but Afrikaans developed from the vernacular Cape Dutch dialect and was later standardized as a separate written language from European Dutch. The notion of Afrikaans being a different language from Dutch didn't really emerge until the early 20th century.
It is probably now difficult for Afrikaners to understand Dutch without formal training because Afrikaans grammar, in some aspects like verb morphology, is simplified/regularized compared to Dutch, and they may not recognize certain inflections. It should be easier for Dutch speakers to understand Afrikaans though (it is definitely much closer to Dutch than, let's say, German).