The "guilty until proven innocent" part highlights a tactic to push back on manufactured claims of genocide paired with smears of genocide denial: genocide is a formally defined crime, just like murder. Just like murder, you start with a presumption of innocence -- that is, you start by denying the accusation, and it is on the accuser to prove what happened.

In cases like the Holocaust (or Palestine today) you have a mountain of evidence. You have countless eyewitnesses backed by film, sometimes video, and almost always official statements or internal documents showing intent.

In China you have significant motivation and credibility questions about the much smaller number of witnesses, you don't have anything like the photographic documentation of the Holocaust, you have some blurry satellite photos of... something despite the U.S. having spy satellites that can read a license plate, and your official statements (that are themselves backed by significant evidence) are about combating radicalization through development.

In short, there is actually a live question about the credibility and weight of the evidence. You do have to engage with the evidence and not simply take the accusation at face value, just like you would at a murder trial.