The systemd service file provided by x11-wm/enlightenment-0.24.2-r1 is not functional by default:
$ systemctl --user start enlightenment.service
Failed to start enlightenment.service: Unit xorg.target not found.
Indeed, the unit file does contain a reference to xorg.target which is not available in Gentoo
you'll have to create that manually even in Arch. Please see https://wiki.archlinux.org/index.php/Systemd/User#Xorg_as_a_systemd_user_service
I've also updated the wiki https://wiki.gentoo.org/wiki/Enlightenment#Use_systemd_user_session_to_select_your_window_manager
I don't think it's possible to fix this with ebuilds.