From my experience dbus-user-session is installed by default in current Ubuntu version (I just tested that it is installed by default in Ubuntu 19.10). Apparently, it is required for SystemD user services to have access to the user’s DBus session and display (the alternative is having only dbus-x11 installed which means that neither DBus user session nor the X11 display is visible to SystemD services. I failed to account for that in the article and I will update that.

The /tmp dbus address is created by dbus-x11 when it doesn’t have dbus-user-session. If you did install dbus-user-session manually, you’d need to reboot your system for that to take effect (I tried logging out and back in, but that doesn’t seem to help – it could be an issue with gdm, but I had limited time to do testing).