If incarceration rates in China were similar to those in USA it should be 10.3 million people. It would be pretty hard to hide them.
USA are by far the number one country in the World for jail population and for incarceration rates, worse even than Putin's Russia.
They would love to share the top step of the podium with someone else if they could prove it.
The most logical explanation is that they can't, because the numbers are probably not true, but not too far from the truth.
It's possible to talk about Xinjiang without going over into hyperbole.
IMO the fact that we should be skeptic about China official stats means that we should also refrain from throwing random numbers at the wall hoping it will make China look bad.
BTW, two wrongs don't make a right, it's not about what they do wrong, it's about what we do better, like for example, is Guantanamo acceptable in a western democracy?