My theory is by the actions of the kings-sorcerers the Athasian tablelands are a demiplane like Ravenloft/demiplane of the dread. Maybe there is a "crimson sphere" or Athaspace with the original Athas planet, but it is practically post-apocaliptic.
Kalidnay was a domain of dread in Ravenloft 2nd taken from Athas.
www.fraternityofshadows.com
Kalidnay was once a large walled city home to Kalid-Ma the Sorcerer-King and High priestess Thakok-An. Thakok-An attempted to ascend her king into a Dragon using the life energies of the city's populace. This, however, ended up putting the king into a comatose state and uprooting the city...
darksun.fandom.com
I imagine slavery in DS but in a more fantastic way. A punishment by the king-sorcerers is to be "muddied", covered with a cursed mud causes the victim to become something like a almost-sentient "drone", with a poor free-will. They aren't undeads or constructs but earth(+water) elementals. The effects aren't totally permanent, according to the "good or bad karma" by the victim. An innocent soul will recover the free will sooner, and then for legal effects the penance has ended. Althought aftereffects are possible. For the king-sorcerers is more practical than the chains and the whip, and for metagame effects, if it is a too fantastic element then it should be softer than real slavery. Officially only the king-sorcerers can be slave-owners (most of times by debt bondage, specially by fault of the taxes) but these can be "hired" by the nobles and rich men.
Othere point is the timeline of the Athasian Tablelands has been rewritten by fault of several factions, agents of Vecna, cultists of Thazridun, and the chronomancers who tried to stop their evil machinations. The facts of the Prism Pentad happened, but within a "crystal sphere", something like a demiplane with a space-time continium, created by the chronomancers to avoid time paradoxes.
Our "Earth" has got lots of "crystal spheres" but they are "isolated by quarantine" because the arrival of the Cybertronians (autobots vs decepticons, the transformers) and other alien factions can cause serious menaces of collateral damages.