That's because the Larian Launcher is not required for multiplayer. You can have only Galaxy running (and start the game via bg3.exe or bg3_dx11.exe) and still play with other Galaxy users as seen in my first screenshot. Cross-play with Steam is only available through Direct Connection which doesn't require either launcher (does also work between GOG users if you don't want to run Galaxy). You can see that in the second screenshot where I joined my own game. As you can see, the Larian Launcher is not required for any of the multiplayer options.

The Larian Launcher is only needed to send Telemetry back to Larian, activate cross-save between GOG and Steam (maybe Stadia? is Stadia still a thing?) and "exclusive rewards" which I have no answer to what those might be.