Haven't read that source, my bad I didn't know that. I do believe that if Athas is within a crystal sphere (and I haven't seen anything that says it hasn't been), it could be approached by a Spelljammer if someone from outside wanted to. I don't know why they would as Athas has almost nothing valuable to someone from another world, and Athas doesn't have the tech or know-how to build a Spelljammer, so functionally it is completely cut off that way as well. It's like you're stuck on a very small deserted island; sure a ship could stop and save you, but if the island is so small they can barely see it, why would they ever stop?
I'll also add they Githyanki once invaded Athas through a portal from the Astral Plane, I forgot about that. Can't remember why they were interested in Athas.
Well if my memory serves correctly, the Githyanki actually try to invade Athas twice. It’s the main idea behind the Black Spine adventure, if I recall correctly.
The first invasion was a simple raiding expedition, like they are prone to do on other worlds. Unfortunately for them, the natives of Athas fought back and proved to be too much for them, so they retreated.
Some of the Githyanki got left behind in the retreat, and mutated into the Gith.
The Gith, for reasons having to do with the environment of Athas, possess greater psionic abilties than their ancestors, and when contact is restablished between the two subspecies after centuries, the Githyanki become curious and rally the Gith into a horde. Their plan is to observe the Gith in action and see if the Athasian Gith’s greater psionics can be weaponized.
Unfortunately, kinda like that one episode of Star Trek Voyager with Species 8472, a band of adventurers from Athas massacre the Gith horde on Athas and then cross over into the Astral Plane where they wreak havoc inside a Githyanki fortress (killing the Githyanki Queen, whose name is Trinth) before returning home to Athas.
In the aftermath, the Githyanki survivors declare Athas to be forever off-limits. The Humans, Dwarves, Elves, Halflings, Half-Giants, Muls, and Thri-Kreen of that world are simply too dangerous.
EDIT
Hmm... Apparently I’m Tetrasodium for Dark Sun.
