On the bold part, there is a necessary and vital distinction. Look at how you are framing it- one Athas. And then trying to place it.
But the original conception is that not only does Athas share the Prime with other settings (and genres, etc.), but Athas shares the Prime with other Athases!
All possible Athases, with all possible histories, are contained within the Prime. So whether you find that more useful or not, it does greatly change the perception of what is allowable and permissible in D&D in the following ways:
1. First, characters are explicitly portable between campaigns. This is something that always used to be assumed, but (for whatever reason) rarely happens anymore; part of the reason is because the conception people have of the setting has changed.
2. Second, there is no single "official" Athas; just the Athas you happen to be playing in. Your Athas is just as official as the Athas of someone else. For that matter, you Forgotten Realms campaign is just as official as that of Ed Greenwood.
3. Further, there is no "canon." All possible Athases have happened, and will happen. Is your Athas hard to get to? Great! But it can be easy, too. Or maybe the gnomes survived and prospered. Or the kender invaded. It all works.
That's the interesting and useful part to it. IMO.