In the article he mentions “Their goal is just to exterminate all Uighurs in one way or another”, if that is the case, why not just shoot them all? Why did they let him out?
Probably harder to claim a dead person is alive, than a live person wasn't tortured. Also it's pretty clear that they don't care about eliminating the people - they want to eliminate the culture.
If I were an evil dictator with the means in the modern day and I wanted to really intimidate a population, I would abduct some of that population and just secretly kill them and burn their bodies. Nobody can then prove whether you actually killed them. You could say you re-educated them and then let them go, and their present whereabouts are no longer your business.