What was specifically leaks is some sort of unfinished build like what was used for E3. While the full campaign and some multiplayer may be available, it probably won’t be playable by normal Xboxes. Only specially modified Xboxes like those owned by developers for bug testing can play it.
Of course for some people, where there’s a will there’s a way. And when there’s a way, there’s also a capture card or PVR so that they can record it and upload it to YouTube to spoil things.