I assume it means a worse chance, personally, but you never know.
Why?
My argument is they saved it for future use. Maybe?
I think you're kind of both right.
This is a very obvious not-Dark Sun. In fact, what it plus "Athasspace" looks like is that, right up until fairly recently, this was going to be the 5E take on Dark Sun.
However, something caused a last-minute (or at least "last quarter of the game") change of plans. So late that maps were made with the old name in place and even distributed digitally and so on. Fyreen almost certainly was Athas, imho. It somewhat much doubt that a huge amount of the text was re-written, because from the pictures, it looks fairly together and well-written, and not some sort of last-minute deal. So the 5E take on Athas was probably pretty different.
So why did they change course? To me there seem two obvious possibilities:
1) They felt like it was a waste to put
any version of actual, named Athas into Spelljammer, because they intend to make an Athas setting, but just not any time soon. This might have been a very recent decision. Clearly they had planned to put Athas in previously.
2) The
specific take on Athas that was previously in Spelljammer was decided against. So, it could be that the Fyreen-Athas was decided as not cool enough, or too wack. Or it could be a more standard Athas was planned, but the book was cancelled or punted into the future. Ray Winninger did say they often cancel setting books (and he seemed to be implying sometimes it was pretty late in the day).
Either way, in the short term, it seems an Athas book is unlikely, because either they've either A) suddenly decided they want a full Athas book where previously it wasn't going to be one, B) decided that the approach for Athas, whatever it was, was wrong, or C) cancelled an Athas book entirely, which the setting would previously have match.
I actually think C is kind of the least likely there. But again, in the short term, i.e. next year or two, I think it's very unlikely we'll see an Athas book because of this. But equally, in the significantly longer term, like 3+ years, the decision not to use the Athas name here means that they have flexibility and could well be planning an Athas book.
If I'm really crystal-ballin' it, like my honest prognostication, then my theory would be, this was essentially a "trailer" for a wilder 5E Athas remake, but they cancelled that, because they want to go with a more standard Athas. You may say "wishful thinking!", but I'm on record as actually hoping for a remake (i.e. similar ideas but different) of Athas rather than a reboot (i.e. same ideas, not merely similar), and I think a reboot is now more likely. Just in 3+ years.
Obviously I may be on crack here.