Furthermore, the old 1e DMG artifacts I'm not sure were pure Greyhawk right from the start, I think they were examples of artifacts for DMs to use as they saw fit. In fact, only 6 of them have explicit Greyhawk references in their descriptions from what I can tell: the Codex of the Infinite Planes, the Cup and Talisman of Al'Akbar, the Jacinth of Inestimable Beauty, the Mace of Cuthbert, and Queen Ehlissa's Marvelous Nightingale. Artifacts like the Invulnerable Coat of Arnd or the Rod of Seven Parts which make references to a background I'm not sure were specifically Greyhawk. Even the infamous Hand and Eye of Vecna don't seem to be Greyhawk specific at that point.
Many of the artifacts first appeared in Eldritch Wizardry (the Unearthed Arcana of OD&D!) prior to the DMG. By Gygax and Blume.
Your methodology is bizarre. This is somewhat akin to someone reading about "Elminster's Staff" and saying, "Hey, since it doesn't say, 'Elminster's Staff, as located in the Sword Coast as detailed in the Forgotten Realms Campaign Setting published by TSR' it isn't really a Forgotten Realms thing." I would find that to be .... incorrect.
Quite frankly, if you are say that certain items (like Heward's Mystical Organ) are not identified with Greyhawk, I don't know what to say. Heck, if you are going to exclude items like the Axe of the Dwarvish Lords, presumably because it was lost in some
unidentified yet specific Invoked Devastation, then I am guessing that we will never have compatible methodologies.
Finally, this does not seem like a particularly relevant or fun discussion to have, since I had started by making an aside as to how there are many artifacts that have a base in Greyhawk (which seems both obvious and unobjectionable, given that artifacts are specific and unique items, and the history of D&D vis-a-vis being written by Gygax), and you want to argue. It is just as true now, as it was then, that the existence of an artifact "of Greyhawk" (no matter how specific) did not prevent anyone from using it in their own campaign. Countless campaigns used these items regardless of their origination; just as, later on, people incorporated parts of the Forgotten Realms that were alluded to in Greenwood's Dragon Articles into their home campaigns without running FR and prior to the Gray Box.
