Well I certainly agree with the point that culling the majority of sentient races from Dark Sun helps add to the devastated feel of the setting. At the same time there is no shortage of diversity on Athas. But most of that life is heavily mutated, having come to exist after the cleansing wars. Gith, Dray, half giants, and arguably Muls depending on the DM are all critters that never existed during the Green Age. And we have a hell of a lot of other sentient races running around, Ssuran, Tari, Aarakocra, Tareks, Pterrans, Nikaal and Villichi to highlight the ones with a proper society. (Kenku and shardminds might also be there, but I've no real idea).
Gnolls are interesting because thri-kreen cover a lot of the niches I would have gnolls fill. A pack based society of savage nomads and pragmatic raiders that aren't picky about what they eat. Hell both are even societies I'd depict as matriarchal as I base both races quite a bit off their respective animal.
I personally however don't see gnolls as a mutant race that came to be with the Red/Brown Age. They strike me more as a minor race that would've taken better to the new world better than most and continue to grow and adapt from there. Since thri-kreen cover a lot of what I would normally want out of gnolls I kind of see them instead as natural trackers. These are the guys who have a talent for finding meat and water just about anywhere. This makes them fine guides, but more importantly to the city states it makes them very good slavers. Perhaps the gnolls came from the southwest beyond Celik and are a relatively uncommon breed to encounter in the Tyr region, at least compared to elves and thri-kreen. But when gnoll tribes do come calling people quickly fear for their freedom.
Also a fun thought. What if flind gnolls in athas were bred by the sorcerer kings as nightmarish gladiatorial monsters?