Yes systemd can use dbus, but plenty of non-systemd systems use and used dbus.

Not true (although maybe in the systemd world?). udev just populates /dev dynamically (rather than having to create all the /dev/ entries manually/via scripts).

Even udev is optional (upon upon a time, adding a new device involved knowing the right 'mknod' invocation in order to be able to connect to it - udev sort-of solves that).

Dbus isn't a systemd project any more than a libc is a systemd project. Systemd just requires it as a dependency. It's a freedesktop project.