I believe that stat is only true for Chinese people that are in prisons and have been convicted of crimes. If you count ALL people being held against their will (e.g. including the Xinjiang "Reeducation" camps), China has more than the US.
China is coy about the actual numbers, but US Asst Sec of Defense for Indo-Pacific Affairs estimates that as many as 3 million people are being held in the reeducation camps (link below). That alone is already more than the total US prison population. Even if you prefer to go with the more conservative estimates (which are mostly around 1-2 million), that's more than the US prison population when combined with the 2 million or so Chinese in traditional prisons.