openvpn.service is mainly a historical leftover. Basically, it imitates
/etc/init.d/openvpn for people who were used to older Debian versions having a single service that controls multiple VPN configurations in one shot, whereas the systemd approach is to have a separate "firstname.lastname@example.org" instance for each individual configuration.
In other words, the service doesn't start anything its own – its only purpose is to aggregate all
openvpn@... instances. If you run
systemctl cat openvpn@.service you'll see that it references openvpn.service, so
systemctl reload openvpn would propagate the reload to all
openvpn@... instances that you have running.
However, note that Debian's custom
openvpn@.service is actually being replaced by the newer
openvpn-server@.service units which come directly from OpenVPN itself. Those two service units are also multiple-instance, but they don't reference the "central" openvpn.service at all.