The box in question is in use as a media server, but reloading logind logs out the X session, so it wasn't really possible to test properly until today. Having the opportunity to restart the machine (probably I just needed to systemctl daemon-reload?) the "don't kill my processes bro" options are working.

The distro is kde neon.

The point isn't that I can't (or the distro maintainers can't) get it to work, the point is that the aggressive systemd default breaks established practice, and that this is typical systemd behaviour: arrogant and uncaring. IOW user-hostile in the fullest sense of the term. It's not like this is an isolated example.