If you call the following:
> busctl tree org.freedesktop.systemd1
you’ll see that, the other D-Bus services depend on the the systemd service …
# journalctl -b 0 --no-hostname --output=short-monotonic | grep -i 'dbus-daemon'
[ 9.637659] dbus-daemon[981]: [system] Activating via systemd: service name='org.freedesktop.ColorManager' unit='colord.service' requested by ':1.3' (uid=0 pid=980 comm="/usr/sbin/cupsd -l ")
[ 9.751952] dbus-daemon[981]: [system] Successfully activated service 'org.freedesktop.ColorManager'
[ 10.625225] dbus-daemon[981]: [system] Activating via systemd: service name='org.freedesktop.PolicyKit1' unit='polkit.service' requested by ':1.5' (uid=0 pid=984 comm="/usr/bin/python3 /usr/sbin/firewalld --nofork --no")
[ 10.710319] dbus-daemon[981]: [system] Successfully activated service 'org.freedesktop.PolicyKit1'
Unfortunately, the systemd D-Bus service doesn’t seem to write a systemd Journal entry when it starts …