I don't see a good reason to think this is going to be set in Greyhawk. I'm aware the 2E sequel adventure, Return to the Keep on the Borderlands, contains a suggestion for where in Greyhawk to place the module, but the adventure itself seems to have been written with Mystara in mind, at least partially. The novelization is set in Greyhawk, and Gary Gygax at some time stated the Pomarj would be a good placement if one was going to set the original module in Greyhawk, but neither the original module nor the video announcement make any mention of Greyhawk. In the video, it's stated that this adventure is set in a place called The Borderlands.