They don't need to be extermination camps to have mass graves, and if I am not mistaken, many extermination camps were thought by outsiders to be prison / work camps until the truth came out.

All of this- on every side- is pure speculation until a trusted independent inspection can be performed. Unfortunately, I am skeptical that will happen anytime soon- I seriously doubt China would see attempts to do so as anything other than a violation of their sovreignty.