It would only take blowing up one satellite to let the world know they're serious about it, at which point SpaceX would almost certainly be forced to comply with China's demands, if only because SpaceX would have to acknowledge that the debris from even a handful of destroyed satellites would cause serious danger to the operations of the rest of the constellation.
All satellite dishes are easily detectable when they are in operation - they have to transmit RF radiation which can be detected and pinpointed to source anywhere from low-altitude all the way up to space. Anyone of a certain age who lived in a country that required TV licenses remembers the enforcement trucks that would detect RF emissions from vacuum-tube TV sets using directional antennae.