USB should result in the creation of a network between the Librem 5 and the host. So synching over USB should be identical to synching over e.g. WiFi. However as a one-off exercise, you may need to configure the network at each end. (Personally I am more than happy to synch via WiFi - rather than having to go physically to the server and connect in via a cable.)
This may be viable if only synching “one-way” i.e. server has master copy and anyone who connects gets a copy of the master copy. I don’t know how well this works if synching “two-way” i.e. merging information in both directions.