Parmandur
Book-Friend, he/him
I mean, it could be inflammatory, so I can see why he avoided talking about it.That's the thing... he doesn't say much at all about what is "between-the-spheres."
Timestamp starts at about 19:20 where he talks Spelljammer. He does say Spelljammer is specifically about the Material Plane (while Planescape is not) which one could assume means the space between spheres is also part of the MP... but what is actually there is not said.