If it predates the MoTP, how can you assert that Gygax wasnt referring to just the Material plane that contains Oerth/ Greyhawk when he made his statement?The two problems with that are that I was explicitly discussing the Gygaxian view (which predates the MoTP), and made sure to note in the OP that the period starting with the MoTP is when it all started to go wrong in terms of the planar structure.
Krynn didnt exist yet from memory. Athas certainly didnt. Mystara was around (in a different system in BECMI). Eberron certainly wasnt. Birthright wasnt either.
Forgotten realms was around though, and it certainly used the 'multiple material plane' model.