For what it’s worth, the authors note that since this is installed on a phone, by the time CellGuard has detected a rogue base station, it’s too late anyway.
https://support.apple.com/en-in/104978#:~:text=If%20Find%20M...
Still lots of devices that have physical SIM cards and would be senseless if the device couldn’t be tracked if the SIM was removed.
I figured it worked more like an AirTag and pinged nearby devices.