haven't debian and other systems been doing that long before systemd? i mean system config files that are controlled by the package manager and not supposed to be edited, but overridden by other files which are to be edited?

Yeah, it's a problem that's not new with systemd. But that never stops systemd from tackling a problem. It's weird to see that they went with a narrow, ugly-hack kind of workaround. And it's weird to see anyone talk about it like a minor but unavoidable nuisance that newbies need to learn to live with, instead of as a problem in need of a cleaner solution.

Yes. All systemctl edit does is provide a shortcut. It calls your $EDITOR variable (as you'd expect).