My Insane Spelljammer Theory
Ok - so I have this massive, weird, a fun theory about how Spelljammer works. This is mostly because I am insane, and have been reading too much. A non-trivial amount of this is based on: http://www.spelljammer.org/essays/history/pw_timeline.html and the scenes in the Cloak Master Cycle where Teldin finds the post-Juna planet. This theory does not match up well if you assume the Dark Sun year length is the same as a Spelljammer year - which I guess is ok - but Athas is always fucked up, but let's accept it.
Ok - my theory is that the Rhulisti are the Juna.
The Mindlords created craft that sailed to the stars using Obsidian Engines in order to power their ships. There is no reason why they were the first on Athas to do that - assuming that this all happened in the Blue Age is of course only speculation
The Kreen sailed the skies longer than any other creatures, including the Illithid and Humans.
The rhulisti were powerful life shapers and therefore would be inventivized to create the strange creatures Teldin encounters.
The Gith of Athas are definitely the origin of the psionically gifted and hardy Githyanki, Githanzerai, and Gith Pirates that go through the spacelanes and the planes. It would make sense that the Illithid came to Athas at some point, and that would definitely be before the Gray came up, or the rise of the Warden Kings (depending on the continuity that you want).
It would make sense that the Spelljammer was created as a psionic, all powerful, race of life shapers
Rajaat's betrayal and subsequent wars would explain why the Juna stopped showing up in space at some point and the addition of the Kreen to space would be explained by the rhulisti being more cautious
Now, obviously - if you look at the Athasian Timeline you will see that the Rhulisti were only 14630 years before the current year on Athas - or at least that is when they began a calendar (I am ok with the idea that a given species developed calendaring before intersellar travel of course). So - we would need to assume that Athasian years are at least 20 times as long as Spelljammer years, or (my preference) assume that the Free States of Tyr have very little access to history, and underestimated the length of a given age. Especially since the Brown Age happened, and therefore, obviously there's a lot of time that the Free Citizens of Tyr really don't understand.
If the Rhulisti are Juna - then Athas is the final cinder of a D&D world that was - the first D&D world, and the first of the cosmos. It is from Athas that all other species spreads into the cosmos of the Crystal Spheres.
Of course, this does mean that The Grey is likely some sort of cosmic enemy sealing Athas away from the cosmos. And that is definitely intruging.