So listening to it now... although they do specifically mention Spelljammer quite a lot, Crawford is actually very carefully avoiding how travel between spheres actually works. He does say you can take your Spelljammer from your sphere to another, but doesn't really say what the space between spheres actually is, or what happens there.
I don't actually know what they're going to actually implement, but re-listening has made me think they will be changing how Spelljamming works between spheres. Not at all sure how, but I feel that if they were sticking entirely to the OG Spelljammer philosophy, Crawford would have just described how it works, with the phlogiston etc. Instead, he pretty much avoids that entirely (he does not say phlogiston, Astral, or anything else really "between spheres"). I don't see why he would do that unless they were considering some changes.