How deep in the weeds are we getting here? Because, me, reading this, it just sounds like a slightly more fantastic adaptation of Aristotle’s model of the universe where outer space was made of Aether

True, Spelljammer's phlogiston is similar to the aether, but IIRC the aether was an invisible medium through which light and gravity propagated. The depiction of a luminous rainbow-hued substance as a medium for ship travel between worlds was new to Spelljammer, IIRC.

I should also note that the use of "crystal sphere", while inspired by Plato's orbital spheres, in this context refers to a shell surrounding individual planetary systems. D&D's Dragonlance, Forgotten Realms, and Greyhawk settings are all in their own spheres. With, of course, routes in between.

Maybe you disagree on the strength of the connection but I think it's neat in any case. :)