It's atrocity propaganda exaggerate by useful idiots, even if out of genuine ignorance. The perpetuator of the original allegation, Zenz, used the label of "coerced labor", which goes through the euphemism treadmill to become forced labor by state department and now slave labor by the even less informed, because that's how propaganda snowballs.
Labour transfer programs in XJ, like elsewhere in PRC are employment programs. Yes there's pressure by recruiters to fill roster. Or it could be part of parole / work release programs. And there may be exploitation, but that's statistically inevitable for a social program that employs 10s of millions from every region of PRC over decades. Also consider these programs are typically well compensated compared to west due to economic disparity of developed and underdeveloped regions in PRC. And they're contracts with limited duration measured in months.
Analogue example is work program that recruits from rural town /rust belt with zero prospects and average income of 20k and paying them 45k to do manufacturing in a more well developed state for a year. Or apply that to work/release for an inmate incarcerated on low level drug charges. Some of these placements are exploitative because capitalism. In aggregate the programs alleviate poverty and promote development. That's the tier of exploitation being misconstrued as slave labour with the XJ propaganda push.