just like the all powerful Celestial Emperor that never was.
Have you read OA? There is a Celestial Bureacracy, headed by a Celestial Emperor. Spirits report to him. There is no suggestion in the text that the Celestial Emperor is, in fact, just a mis-named Parochial Governor.
EDIT: missed this earlier
It can't be at the meta level. AO explicitly says there is a western world and players can play western races and classes as Gajin. That's canon. That's RAW. That's not meta at all. The entirety of the three sentences are in-fiction and explain the thematic differences between the western world and eastern world.
There is nothing in AO canon that says the Celestial Emperor is even a god, or rules even the eastern gods, let alone the western ones called out in the setting as separate from the eastern beliefs and way of doing things.
OA says there is a Western world. It doesn't say that the gods of that world are more than petty godlings subject to the rule of the Celestial Emperor (should he wish to assert it). It doesn't say that the Western conception of the world, as unordered, is true - either in general, or even where those people come from.
Re-quoting the passage from OA p 116:
Unlike the western world, which has always tended to view non-human creatures as a loose collection of beings with no unity or cohesion, the Oriental mind has organized the world into a unified whole. One particularly strong belief is that of the Celestial Emperor, a powerful being who heads the Celestial Bureaucracy, a type of government of the spirits. Many of the spirit creatures described in this section come under his command and many hold offices or positions within the Celestial Bureaucracy.
It can't be true that the world of Kara-Tur is a unified whole, but when you cross some boundary (which one?) the non-human creatures lose their cohesion. (Except in so far as they haven't yet been subjected to government.) Or - and to borrow from real-world geography to make the point - is the Chinese coast of the Pacific Ocean ruled by dragons, but the Californian cost not? What about Hawaii?
Equally, it can't be the case that the Celestial Bureaucracy is mere belief,
and yet it has spirits as office-holders. Clearly, if it has officers (in the form of spirits) then it exists, and the belief in it is true. Which means that the non-human world is not a mere "loose collection of beings with no unity or cohesion".
Either the world is subject to a Celestial Bureaucracy of spirits and the like, or it's not.
No one playing a Planescape or MotP game supposes that the elemental planes exist only in relation to certain mortal lands. Either the world is made up of those elements, or it isn't. The Celestial Bureaucracy is in the same boat.
As to whether the Celestial Emperor is a god - as far as I know the only treatment of him in original AD&D is in DDG, where he is Shang-Ti, the supreme god of the Chinese pantheon. Seems fairly god-like. In OA itself (p 122), we are also told that the Hu Hsien (fox spirits) "greatly fears thunderstorms since the Celestial Emperor sometimes sends the Thunder God to punish the hu
hsien for its wicked ways." Generally speaking, only gods command other gods. And, on p 127, we are told that "Go-zu Oni, and the me-zu oni, form the bulk of the Celestial Emperor's army in times of trouble and insurrection. They also oversee the lands of the dead and escort the reluctant departed there." Again, I think it is fairly clear that the Celestial Emperor is a god, given that his army is made up of the guardians of the land of the dead.