Hmmm. I have another note that "Alibama" comes from a textless tune, "Daar Kom Die Alibama" re sighting CS Alabama by Malay fishermen off Cape of Good Hope. Marais added the "corect" words. (The ship was "Alabama." The song was "Alibama."
As to their background, I'd always assumed Miranda was a Jewish Holocaust refugee (from her name and the several bios that she fled the Nazis) and that Marais was also Jewish - from his and his father's names. I don't recall anything about his mother, though. I suppose he could have been mixed heritage.
He was Josef Pessach and she Rosa Lilly Odette Baruch de La Pardo de Miranda. (The Netherlands was the great refuge for Inquisition-expelled Spanish Jews.) I don't think he would have been permitted into the Cape Town music academy or to join Symphony Orchestra if he'd had any "colored blood" at all.
As to his politics, I agree that compared the New York activists he was pretty mild. On the other hand, he was raised in the bush & heavily influenced by (and apparerntly spoke) native (Hotentot & Zulu, anyway) customs and world-view. Miranda was a refugee, of course. I think the message of their music was loud and clear - Universalist, egalitarian, one-world. He was among the first to bring S.African tempos & ethos to America and the great bulk of their songs were "World" songs. Never exploited, always explained so the audience would understand the setting and customs of the people that originated the song.