My point:
There are no native born Homo Sapiens Sapiens in southern Africa who carry Neanderthal or Denisovan DNA.
Outside of southern Africa, there are definitely Homo Sapiens Sapiens who have Neanderthal and Denisovan DNA.
Why are those two distinct groups of Homo Sapiens Sapiens considered to be part of the same Hominim species?
They all branch from a common ancestor that originated in Africa and migrated out from there. Homo species migrated out of Africa LONG before modern humans, including the Neanderthal.
Our branch is relatively new and survived, the others died out. When inter-breeding was possible it apparently happened because traces of the other hominid species DNA can be found, but the species it came from is no longer around.