In the sense that for Europeans in the 12th century “the world” was limited by their knowledge, yes.
But some players and groups cannot accept the limited logic of a world that is not part of the celestial bodies that float in space. The limited knowledge of PCs is one thing. But if we want to imagine a world that makes sense to us we must go beyond the simple perceptions of PCs.
It's like what science fiction authors wanted in the 1950s: an accurate representation of fantasy using science and logic
— Joshua Morris (@the_grezkev) January 30, 2019
Many realms of existence in D&D and other fantasy settings are literally not planets.
— Jeremy Crawford (@JeremyECrawford) January 30, 2019
I’m aware. I’m saying for some that isnt enough.
Some people may not go the length to realize Forgotten Realms (the most popular realm) is on a planet. Some may not care. But for those who do, they can learn about it.
Players can be thirsty for knowledge. As DMs, it is a duty to provide a world that fits the imaginations of their players.
— Joshua Morris (@the_grezkev) January 30, 2019
DMs are definitely free to customize D&D’s many realms.
— Jeremy Crawford (@JeremyECrawford) January 30, 2019