Using their army for phase III testing is perfectly reasonable in a society like communist China. They are there to serve the state, with their lives if necessary, and the state's biggest need at the moment is subjects for a phase III trial.

Doctors wouldn't consider it ethical in the West, but in our armed forces people get "volunteered" for hazardous stuff all the time. And it's probably not very hazardous, since they already finished phase II. I bet contracting Covid-19 is a lot more hazardous.