Dark Sun went out of its way to subvert most of the tropes around, which is why you get cannibal halflings and thieving, grifting nomadic elves. The dragon was just another one. As stated, it wasn't your typical dragon. And could kick the butt of any regular dragon anyway. The Valley of Dust and Fire, which covers Ur Draxa, the city where the Dragon lives, point blank says you should never be able to defeat it. In fact the whole module is pretty much the most impossible fortress ever released by TSR.

Dragon slaying weapons are noted to actually work on dragons though, so it is, technically, classified as a dragon.

And yeah, there is at times some disconnect between different rule books about descriptions and history and what not. This was because at that stage TSR was in terminal decline and was desperately releasing as much stuff as quickly as they could to try and recover. (Spoiler: it didn't work.) That meant the various writers of modules often had only a limited knowledge of the setting they were writing for and having to write quickly with limited editorial oversight to insure consistency.