Sure China is good at censorship, but not even North Korea is that good. 9m bodies in under 6 months? China can hide mass graves, vast amounts of incineration, or millions of people missing, but can't hide a handful of demonstrations? China is not a black box to that level. Even in WWII before cell phones, internet, and satellites there were more substantial clues of mass deaths.
It doesn't matter that China has a billion people. Most of these deaths would be concentrated. You can't really hide numbers like that. If China was able to accomplish that kind of deception, that accomplishment would be more noteworthy than the virus itself.
There's little doubt China is underreporting. But I'm not buying any algorithm that spits out 9m.