You can't do this with systemd. A kill to a process group is an atomic operation in Posix so that if you do a kill -9 -1 (i.e. send a SIGKILL to init and all of its children), the kernel will not return from the "kill" syscall until it has sent the signal to all of the processes. That syscall will also prevent any other task switches until it is done so the result is no process (other than init) ever runs again even if they are in the middle of a forkbomb. A kill -1 -1 (send SIGHUP to everything via init's process group) has traditionally told all user level programs that the user logged out and all daemons that they should reload their config files.
Killing a process group (the negative process id, which is what the original commentator was talking about, not a SIGHUP) is used all the time on systems. That is how apachectl (and most other forking deamons and their control programs) tell its children to reload the config file or end in a controlled way. It is used every time a user logs out to make sure all their processes do go away. Signals are the oldest and more reliable of the IPC mechanisms and are great when the number of messages you need to send is a tiny number of options.