Na página do systemd que você postou:
How do I change the default runlevel?
systemd uses symlinks to point to the default runlevel. You have to delete the existing symlink first before creating a new one
rm /etc/systemd/system/default.target
Switch to runlevel 3 by default
ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
Switch to runlevel 5 by default
ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target
systemd does not use /etc/inittab file.