Guest user accounts can be created and registered with the hub by an admin user, as the link you posted explains. It doesn’t matter where the guest user is when that occurs.
Similarly, they don’t have to be connected to your LAN at all to login to the Hubitat mobile app. While on an isolated guest network, they won’t be able to use local dashboards without appropriate firewall rules though.
The mobile app’s presence sensing uses a geofence, which could be set, for example, with a radius that far exceeds your home’s WiFi coverage. So again, doesn’t really matter whether a phone is on a guest or trusted SSID.