I mean who cares what the MOTP from three editions ago says?
IIRC the 3E Manual also said, "The Prime Material Plane of the D&D cosmology is called Oerth."
obviously that's not how it worked before or after 3E. Oerth isn't even the only planet lit by the same sun, much less the name of whole Material Plane.
3e MotP recognizes Oerth as an individual world but also gives it as the name of the 3e default Prime Material Plane. The latter is new to 3e as far as I can tell.
3e Manual of the Planes pages 5-6:
"The core planar arrangements are provided as an example, and you should choose which parts to keep and which parts to create anew.
The center of the Great Wheel is Oerth, the core world for the D&D game. Around it lie the Inner Planes of fire and water, earth and air, and positive and negative energy. Beyond are the planes of good and evil, law and chaos. The white mists of the Astral Plane connect it all."
Page 16:
"Material Plane:
The Material Plane encompasses the world of Oerth and the world presented in the core D&D rulebooks. Alternate Material Planes may exist as well."
Page 42-43:
"
OERTH: THE D&D COSMOLOGY’S MATERIAL PLANE
Oerth, the Material Plane for the D&D cosmology, consists of self-contained spherical bodies hanging in space. Oerth has all the usual traits and connections of a Material Plane. It connects to the Astral Plane, the Ethereal Plane, and the Plane of Shadow, and travelers from Oerth may reach the Inner and Outer Planes through the Astral Plane.
Oerth’s cosmology is laid out in the planar diagrams presented in Chapter 1. A sphere of the six Inner Planes surrounds Oerth, which also connects to the Astral Plane, and through it to the Great Wheel of seventeen Outer Planes. Each of the six Inner Planes is not connected to another Inner Plane, but each Outer Plane is connected to its neighbors. If you know where to look, you can find your way all the way around the Great Wheel from Celestia to the Abyss and back.
In addition, there are more small demiplanes connected to Oerth than can be counted. Many of these have been created by deities or powerful wizards as places to hide their riches, get away from the mundane world, or imprison particularly unpleasant enemies.
Oerth does not have any known connections with alternate Material Planes (see below). These alternates may exist intermittently if you want your campaign to visit them.
While Oerth itself does not have elemental or energy traits, particular locations on the plane have pockets that have elemental traits. These areas are often located near portals or vortices to the various Elemental Planes."
For purposes of Snarf's original multiverse that second to last paragraph is key "
Oerth does not have any known connections with alternate Material Planes"