The truth of that statement is dwindling rapidly with time, however. XPC is closed-source, and with launchd being subsumed into it as it has become more importa

The truth of that statement is dwindling rapidly with time, however. XPC is closed-source, and with launchd being subsumed into it as it has become more important to macOS's system, Darwin hasn't had a working init system for years. It's been effectively TiVo'ed, or turned into shared-source-ish instead of open enough to run it as a real OS.