I think that you are confusing two separate declines. The fall of the ancient, probably Yemenite "Zimbabwe" took place many centuries before Marx, or the earlier egalitarian lunacy in the French Revolution. That the civilization mentioned, here, was probably of a people closely related to the Yemenites is suggested by the peculiar stone architecture, which closely resembles formations found in the Yemenite region of the Arabian Penisula.
There is no real evidence that any of the peoples presently in Southern Africa had much to do with its creation or its fall. (There is the possibility, also, that this had some connection with the legendary "King Solomon's Mines," which is not inconsistent with the Yemenite theory; as we know Solomon had connections with Sheba in that region.)
William Flax Return Of The Gods Web Site