So HK police are required to wave a blue flag so that the general public is aware the police have declared a particular gathering at a particular location illegal. Thus, HK law literally wants the location of the gathering made known, publicly drawing attention to it and the location. I assume this is to inform those gathering about their status and to let others nearby know also, as a warning, perhaps. And also nearby officers. Okay.
So consider an app that shared only this publicly visible information on a map. i.e. "police have raised the blue flag on this gathering, right here", without commentary or calls to action.
Less functional and informative than the existing app, but it may still serve a useful purpose for HK residents and demonstrators and perhaps even police (according to HK law) and then Apple may plausibly allow the app because it doesn't get into choosing sides or facilitating illegal calls to action.