If the connection between the two devices shows up as a network card or something similar you can block syncthing to go on “internet” using a firewall so to restrict it to be able to reach only the IP of the other device.
If the connection between the two devices shows up as a network card or something similar you can block syncthing to go on “internet” using a firewall so to restrict it to be able to reach only the IP of the other device.