I’m sure it’s one of their sources though. They’d be silly not to take advantage of publicly-available data. I’m also sure they don’t rely 100% on ADSB Exchange for said data. It’s just one of many sources of intelligence that their analysts use.
Other than if there’s, say, a massive air launch headed toward China I would think they’re more concerned about being able to predict America’s long-range plans and activities. Tracking what planes are going where and where they came from is just one small piece of the puzzle.
It may well be that the US wants China to have this data for reasons of their own, otherwise most if not all military planes would be ordered to fly without transponders operating, or the military would develop their own encrypted tracking system rather than using the in-the-clear civilian version. (They may actually have this already in addition to the civvie ones for all I know.)
In any event, it’s foolish to think that not feeding a particular flight tracking site is doing anything positive to hinder any country’s intelligence operations. They’ve been in the game since long before there were any such things as Raspberry Pis, SDR dongles and all that.