I was going to say that it signals launchd's process group — every process spawned by launchd, which always runs with PID 1. However, the `kill` manpage confirms your hunch:

  -1      If superuser, broadcast the signal to all processes; otherwise
          broadcast to all processes belonging to the user.

(This is on macOS/iOS, Linux might have slightly different semantics.)

Yup. From man kill:

    -1

    All processes with pid larger than 1 will be signaled.

Yes, but not for the current process and init

To all with a PID > -1