[marios]

As pointed out by JdeBP, I confused RemainAfterExit= with KillUserProcess=. RemainAfterExit is an option indicating how systemd should process a unit file, while KillUserProcess is for systemd itself.

It's nice to know that libsystemd is not required in all cases, but I still have a problem with software having to be modified because systemd changes how libc's daemon() function behaves.