I'm not sure if he's arguing there are extralegal, "off the books" detainees (his story isn't very clear). I think he's arguing these people are accurately counted by China, but that (1) the process for arresting them isn't fair and (2) something sinister -- a genocide with no mass killings, or a "cultural genocide," or maybe just mass human rights violations while in custody -- is being done to them.

His "insufficient due process" claims come solely from China's (supposedly) high conviction rate, and are a frankly bizarre angle to focus on if one is even flirting with genocide accusations as well.