But those comments weren't just about location - everyone knows that triangulation based on cell towers is a viable option as long as you're connected to some. But they also claimed that airplane mode, which is supposed to disable most communications modules in your phone, including the cellular modem, would be ineffective at doing that. To me that seems to reach into "the US government can remotely turn your phone on" and similar kind of theories.

As for burning - if they really possessed these extra special exploits that allowed monitoring of even supposedly disconnected or disabled devices, each instance of its use would expose them to a slim, but nonzero chance of that exploit being discovered, especially if it required communicating with that phone directly. In this situation it would be wise to limit the use of this to actually important targets, to avoid revealing their advantage by using these unconventional methods (as opposed to normal cellular, wifi or GPS-based tracking) on random protestors.