That's a common misconception because the Bantu expansion didn't reach the Cape. But we have iron artefacts from Natal dating from the 400s.

I'm open to learning more, but that doesn't seem like very strong evidence. Iron artifacts reached e.g. Mongolia long before anyone there was capable of making iron. Iron is very durable and very useful; it is one of two materials that is, as far as I know, universally traded for in every known culture that might conceivably have had access to a trade network featuring the object(s), the other being salt.