That’s not an exit code; that is a fake exit code which your shell made up, since your shell has no other way to tell you that a process did not exit at all, but was killed by a signal (9, i.e. SIGKILL). This is, again, not an actual exit code, since the process did not actually exit.
See here for why you can’t use an exit status of 128+signal to fake a “killed by signal” state, either:
