The Space around a planet is Material, but if you get far out enough (Wildspace) it becomes both/porous. Spelljammers are for both space travel and planar travel.
I genuinely don't understand why, but okay.
I justified it by being Resisting Dark Urge, and thus convinced my tadpole was somehow worse than everyone else's, and willing to go to nearly any extreme to get it out. I even sprung Sazza from the grove (after the Alfira incident) because she thought Priestess Gut knew of a unique way to handle it - but discovering that she herself was tadpoled, I abandoned that avenue.