Exactly. So why is it part of your init system? Can you think of a reason why it is not a totally separate project which is totally independent of systemd?
From a surface glance systemd and openrc are almost identical. Both require very little to get something to work. The difference is that systemd does a million things more.
What you probably should know though is that many systemd distros do not have a real fstab, the fstab is just a file which systemd uses to create mount units, which are then used during boot.
Another important piece of knowledge is what a systemd "transaction" is, that is a very unintuitive concept which you absolutely need to know if you are debugging on your init system.