There are many diseases for which the susceptibility varies greatly among ethnic groups, due to differences in genetics and point mutation distribution. For obvious reasons, there is no "perfect" isolation. For a cheap example, there is the well-documented genetic resistance/immunity of some northern European caucasians to HIV. Protections against infectious disease like this are never universal for an ethnic group, but it does generate significant differences in transmission and infection rates.
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.