Which you appear to have not read or understood, so let me make it easier. Covenant theology is pretty simple, there are only six biblical covenants. Three have been fulfilled (Abramic, Noahic, Mosaic). The remaining ones (which continue, and are not yet completely fulfilled) are Adamic, Davidic and New. Of the remaining ones, the first addresses all mankind, second addresses the Jews, and the third also addresses all mankind.
Exactly which of them address the "Afrikaner" people exclusively in support of Buchan's statement? This is the key question, because Buchan purports to be an elder, and as an elder needs to be able to teach the truth from holy scripture.
Seeing as all scripture ended shortly before the destruction of the Temple it would be impossible to specify any further covenants would it not? Also seeing as this particular covenant is of no matter to anyone other than Afrikaners why would it be included in scripture?
To believe in any set orthodox theology without question is an intellectual trap, the same trap the Sadducee's, Faracee's, Catholics, Orthodox, NGK etc fell into. Tradition becoming dogma never ends well.
The NGK Sinod in the 70s and 80s clearly made it part their (flawed) doctrine. The "likeness to Israel" at that time (ostracisation from the rest of the world, drought, warfare) was exploited to indoctrinate the "Afrikaner" people in the belief that they were specifically shared in the covenants that God had with the Jewish people. This was the basis for Apartheid, which was justified in the light of the fact that Jewish people were prohibited to marry people of other tribes (in order to keep the bloodline clean for the saviour, Jesus Christ, in fulfillment of the Adamic, Davidic and New covenants).
Again, I have to ask you, exactly which of them address the "Afrikaner" people exclusively. What is it that you are defending, if not Buchan and his theology?
Which actually proves my point, the theological ideas were older than that decision and the decision only happened because of political pressure. From the very start the NGK opposed the great trek, the war that blood river was part of and the covenant itself, then when the political winds shifted they suddenly endorsed it..... it did not start with them.
Buchan misused the emotional power of the covenant in his favour, it's not him I am defending. He has proven to be either a fraud or a coward, I would have thought I had made this opinion clear by now.
There was more than one basis for Apartheid, what you mention here is just one of many and chiefly used for propagandic purposes, it was not the main one. You start your history too late and ignore too many things.