Can you cite sources for this? I ask because
Defilers and Preservers: The Wizards of Athas (1996) says (in the "You Can't Get There from Here" sidebar on page 10) that Athas is in its own crystal sphere, which is impenetrable to spelljamming, and that the presence of the Gray makes it very difficult to planeshift into or out of the sphere. This information is in keeping with what's written in
The Planewalker's Handbook (1996) - which was written by Monte Cook - about Athas (page 32). Likewise,
CGR1 The Complete Spacefarer's Handbook (1992) equivocates about Athas's status, simply saying (pg. 13-14) that it's either unreachable or simply very far away from known space.
Insofar as I'm aware, the best argument that goes the other way comes from
Dragon Kings (1992), specifically "The Planes Beyond" section on pages 64-67, and even then it strikes me as being less unambiguous than a lot of people seem to think. Specifically, it says:
A lot of people seem to take that second sentence as a declarative statement that Athas is on an alternate material plane from the standard cosmology, but I think that's a misreading. Rather, that sentence seems to simply be noting that there are many alternate material planes (one of which has Athas), and is silent with regards to whether or not this is the same one as the "standard" D&D multiverse.
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