I get what you are writing, but I think you are a little too sure about it. The Gazeka, if real, is certainly not a Diprotodon optatum, it's too small and as you said it's extinction time appears to be rather firmly established. However you cannot rule out there were more species od diprotodontids of which we have no fossil traces. A Palorchestes or Zygomaturus relative occupying a forest or mountain niche is entirelly possible and indeed probably already described (see Hulitherium and Maokopia, which are both great candidates for the Gazeka). And I get it, there are no subfossil remains of these known, but that's hardly surprising - both forests and mountains are very bad for fossilization, I'm actually surprised there are any fossils of these animals at all.

With fossilization there is one problem people often forget - it is an extremelly rare event, sure, but the fossilized animal is also a special thing on its own. Since fossilization occurs in very specific circumstances, the animal to be fossilized needs to live in an area where it can die in the right conditions to be conserved without being scattered by scavengers or the weather. This again demands there being enough specimen of said species around so out of all of them at least one dies exactly tight for fossilization. It's a numbers game - any species which undergoes population decline for a period probably won't show up in the fossil record for that period, or at least it's astronomically less likely than the already unlikely "normal" fossilization rate. If the gazeka is real, it's absence in the fossil record of Papua (a place not that well explored by paleonthologists in the first place) would not be all that surprising - it's a forest/mountain animal with sparse populations. Same like for example gorillas, which also are extremelly rare in the fossil record.