The extremely generic product description leaves me little room to discuss details without violating my NDA, but I think I can safely confirm that a significant section of the adventure takes place in the City of Greyhawk. Provided you are willing to 'port the Free City into your campaign, you could pretty easily run it in any campaign setting, but come on. It is written to be used in Greyhawk or as a stand-alone, primarily. I certainly didn't spend a lot of time thinking about how to plug it into Eberron or wherever.
And while there are some humorous elements to go along with Zagig's madness, it is not a joke module, and we did not bother to consult the joke version of Castle Greyhawk when researching the book.
We did include several elements from "Greyhawk Ruins," but that is a deeply imperfect adventure, and we certainly didn't follow it slavishly.
--Erik