That’s a different kind of message, a so-called RRLP (radio resource location protocol) request. Those are what ask the phone’s baseband “hey, give me your position”, which then takes its measurements and replies. There are 2 ways of doing this:
-GPS (assisted, normal, or just sending the raw received data back along the line for the other end to process) - something which won’t be possible on the Librem 5 unless you, the user, explicitly allow it (because the modem can’t talk directly to the GPS).
-Time of flight detection, where all the base stations in the area broadcast a message at the same time, and the modem records how long it takes for each one to arrive. If you can only see one base station (either because you’re in the middle of nowhere, or because you’re using highly directional antennas), all you get is a large circular area rather than a single point.
Silent SMS messages (the so-called “empty paging”) actually won’t be affected by highly directional antennas, but for a different reason. All they do is cause a location update, telling the network which cell tower you are attached to right now.