I'm aware and that's a bug, not a feature. If you are online and go Offline with the explicit button (instead of unplug the cable) and actually keep the cable connected (not sure if LAN or internet at this point) and start a second Steam client and use the library (not sure if online or offline for the second one), the offline one will not be able to use the library because already in use.

The workaround is, indeed, pull the cable or firewall steam, go offline, plug the cable.

So the concern becomes "what if the protection becomes stronger".