Ok, it doesnt have a kernel full of hardware drivers or an implementation that runs on bare metal. That said, it covers a lot of userland and things otherwise covered by a Desktop Environment (which are often conflated with OSes). In terms of application hosting, window management, low-level system control, over etc. and the fact that some brave souls have had success with running emacs as PID 1 means that it's not a million miles away from being an OS.

(And that's without the inevitable implementation of some basic OS functions in elisp by various wizards)