I doubt anyone can truly answer that question, but I’ll tell you both what I’ve heard from people who worked on Dark Sun and what I can find looking at clues:
On an episode of the Bone, Stone, and Obsidian Podcast, one of the interviewees was the head designer for Elves of Athas, an early Dark Sun supplement, who confirmed that he had no idea whatsoever of the true history of the world or even a lot of details of the city states. Similarly, the Dune Trader sourcebook has a pile of infamous continuity errors. Really, few Dark Sun products managed to be internally consistent, much less consistent with the rest of the setting. (Good example here: Earth, Air, Fire, and Water claims in 2 almost back to back paragraphs that the paraelemental planes of Athas are the same as the ones of the Great Wheel and that they are completely different, which is it?)
Even the head designers, Tim Brown and Troy Denning, seemed to have communication problems, with books worked on by one (ex. Dragon Kings) being heavily contradicted by the other (ex. parts of the Prism Pentad). Or just look at the description of the box set Dragon of Tyr compared to Dragons Kings compared to Prism Pentad. According to Troy Denning in an old interview, they could never even agree on whether or not there were once gods on Athas, or the number of Champions of Rajaat.
There are other weird things too. A truly old dragon magazine held Psi-Shadows, a monster designed to be a call forward to the upcoming Dark Sun campaign, that feels a bit like a very early version of shadow giants, but works completely differently in all respects. Or how about the many monsters in the monstrous compendium for Dark Sun that were never even given a reference in the Prism Pentad.
As best as I can tell, the truth is that there was a lot of disagreement, things just thrown in there, and, most critically, a lack of open communication between people working on the setting from the very beginning. I don’t believe the Wanderer was meant to be unreliable, at least not to the degree that he looks with the benefit of hindsight, but the secrecy shrouding the setting was not just targeting the players, or DM’s, but the designers themselves.