I use LightDM with XFCE, and the "Switch User" option is disabled.
I tested the following fix, and it works perfectly:
https://wiki.archlinux.org/index.php/LightDM#User_switching_under_Xfce4
I think this is a bug in Fedora's packaging. Either xfwm4 should be patched
to look foreither gdmflexiserver or dm-tool, or gdmflexiserver should be a
wrapper that looks at where the display-manager.service alias is pointing
to, and invoke the appropriate switcher.