The majority of any OS lives in user space though. Intercepting syscalls is also not that weird of an idea, that's how tools like strace works. Building out sufficient kernel functionality without needing to forward calls to the kernel is definitely impressive though.
What do you mean by that? There's a notion of an "operating system" that encompasses both the kernel and all the userland tools (in this sense, each Linux distribution is an "OS"), and there's a more common notion of an OS that is just the kernel and any userland services required for the kernel to function; the latter is the more common definition.