Unlike you, these folks don't exclude indian, asian and tribally disenfranchised black / brown South Africans from the convention.

Any South African is free to join and unite with them against governmental mismanagement, overreach and bullying.

Your whole argument also seems to be completely (and incorrectly) based on whataboutism. Yes, they focus on the Afrikaans community. That is their niche focus and a focused lens is an infinitely better starting point than dealing with unfocused malaise.

This does not exclude other communities. Assuming that is simply lazy logic. Supporting and collaborating with other community centric groups is central to their ethos and operations.

If you still don't feel specifically represented enough, do something about it. Like they did. 

Get a grip, get organized.