That is how I understand it: If you comment it out the warring will go away and the systemd-fsck will not work. You can verify it by running:
systemctl status systemd-fsck-root.service
I am reading more on the subject and as it looks like it all goes to the “Systemd does it all” , I am trying out the systemd-fsck, so I left the grub line, removed the fsck from the HOOKS line in the /etc/mkinitcpio.conf and run:
sudo mkinitcpio -P && sudo update-grub
The systemd detected my filesystems, created services for them and runs the scan on boot.