I could not have said it better.To add on to this. Historians rarely know the truth, but most of the time just have to assume probabilities and likelyhood. And it's very likely that every civilization, be it advanced or not, to have philosophers who didn't write down their work or had it destroyed. So assuming these things says very little.
What makes historical philosophy interesting is not that it was contucted, but its content. If there is no content, there is little value. Such is the early history of SS Africa. Unless you interpret rock paintings a philosophy in any way.