Jamming is merely intentional, targeted interference; if something else is using that frequency, the satellite is already effectively jammed from the interference caused by other users (aside from some advanced frequency sharing schemes).

You and that other poster are probably thinking of the traditional sense of jamming, which interferes with a large range of frequencies. That's "traditional", in the sense that it's cheaper, easier, and works better when the target is capable of avoiding jamming. In this case, none of that applies. A nation state like china can easily jam a specific narrow band of frequencies without affecting neighboring users. Which is exactly what's happening to GPS in many areas.