The rift of Aln is a gate connected to many dimensions, is not that the emerald dream is connected to outland, is that the Rift of Aln is connected to the twisting nether.
I think the emerald dream is a pretty important part of the lore to simply not mention it, if in the future they go and say the emerald dream spans across the universe, it would feel like a plothole if the titans never showed signs of this being true.
- - - Updated - - -
yet all those "suppositions" start with the notion that the dream is an azeroth thing, whether its Freya creating it or weaving it from Azeroth dreams, the fact that it resides on azeroth is implied in both statement, so the logical conclusion is that the Dream is an Azeroth thing.