Hi Caiman27,
Yes those airports have non-stop service to all continents but that is operated by their own airlines. For example, EK flies to all continents from DXB.
What I am saying is that at least one native airline from every continent operates a non-stop service into JNB.
DXB is not served by a South American carrier.
DOH is not served by a North/South American or an Australian carrier.
Even LHR does not have non-stop service from QF but rather a direct one via DXB.
Put another way, from every continent there is at least one carrier that starts their service there and ends it in JNB without any intermediate stop along the way.