Why doesn't this sound likely ? Because we've spent all our lives hearing how it's impossible, yet we have absolute physical proof that this was already happening before it was interrupted by colonialism - particularly as it peaked in the 19th century.
And even where it was acknowledged - it was covered in propaganda. The Difekane we are told - the march of bloodthirsty barbarians killing their peaceful neighbours...
Rather an odd judgement from the bloodthirsty barbarians who were killing those same neighbours, and the Zulu's and taking over their lands.
And how different this view of Shaka's emerging Zulu empire  - when compared to how we view European empire builders like Alexander, Charlemagne, Gustavos Adolphus or Charles XIII. We honor them and forget their bloodthirsty cruelty (Charles liked to skin PoW's alive).

Yet there is no real difference between Shaka and Alexander. Between Mzilikaze and Charlemagne. Between the Difekane and the rise of Rome.