While plausible in theory, it would be nigh impossible in practice, particularly when trying to target a significant diversity of ethnic groups to the exclusion of one. Doubly so for targeting caucasians, with their unusually filthy and diverse genetics. You are much more likely to get a human planet-kill out of it than an ethnicity specific disease, and no matter what kind of cleverness they come up with, quite a few Asians and Chinese will get taken by it.
While susceptibility to diseases is statistically biased in a fashion that follows ethnic genetics, it tends to follow very narrow genetic profiles -- you cannot tag a broad genetic swath of humanity except with the crudest filters that will miss a hell of a lot. And as I mentioned above, it would probably be unwise for the Chinese to start that game, as caucasians generally have far more robust genetic protections against pandemics than ethnic Chinese. Even if they were able to create such a thing, there would be an awful lot of caucasians left over with excellent biotech skills and a much more vulnerable ethnicity as a target. Dealing with a biological first-strike scenario of this type would be super-ugly, but by random luck of genetic history, there would be a significant asymmetry in vulnerability and the Chinese who developed these weapons would know that.
Fear of retribution is a bitch.
Correct. And anyone who believes we don't have our own 'payloads' we could deliver to China is sure dreamin'.