Could the ChiComs possibly be betting the farm on sheer population sizes here? Try to imagine our economy with a 40-70% death rate. They have an essentially new military, that is isolated from the masses in China - keeping them protected as a group.
Once the infection hit here, we'd have mass panic, quarantine, some anarchy. I don't know how far that anarchy would go. We'd fight quarantine - since being quarantined we'd have to trust our keepers to not just turn to the "final solution". I know I'd fight it.
Russia would be decimated, since this is already there, and they've got negative population growth big time already.
India's health care system would be overwhelmed - to say the least. They'd lose at least 70%.
Is the CCP willing to gamble on this - now??? Their window of vulnerability will be closing for nuclear attack once our missile defense is online.
The US is simply too genetically diverse, buttressed by a strong northern European genetic component. Anything that is strong enough to hit a 40% death rate in the US will also be strong enough to sweep through China.
Also, China is a filthy and crowded third-world country with very marginal sanitation and an economy that is FAR more dependent on human labor and interaction than the US is. The transmission paths will be much denser and quicker in China than in the US, and they would have to squash a much bigger part of their economy to have a prayer of containing it.