If I recall correctly I don't think that ZS-OLD had GPWS on board.
I know of only 1 1900 in SA that has EGPWS.
As far as I know in SA if GPWS is installed onboard then it must be servicable. If the 1900 arrives without it installed I don't think there is a requirement in SA to install it.
Secondly EGPWS relies on a Database of airfields. Many of the airfields operated into in Africa are not in the database. Hence the aircraft have to operate into some of these fields with terrain inhibit on to avoid nuisance warnings.
Thirdly if ZS-OLD had the gear down the only warnings they may have got if GPWS was installed would have been "Sink Rate" or "Too Low Flap".
Sadly if a 1900 is operating in that area GPWS should be a serious must have. But then comes commercial pressures to get the first available aircraft out on the contract asap etc etc