Huh. My impression – that it is the same Astral Plane for all campaign settings – came from the
Sage Advice on Planes and Worlds 7/23/18.
Well, as I said... my opinion of "multiple Nine Hells" was coming at it also from a metagame perspective.
The meta perspective being that every single real life player runs their own game and that nothing any of us do impact any other player and their game. There could be 100,000 DMs all running Tomb of Annihiliation. Every single Chult realm is going to thus have their own story and their own plot. Some NPCs might die, some won't. Every one of those Chults will be slightly different, but every single one will also be "real". Anything that happens in our games is "real" from the perspective of our games and the perspective of D&D on the whole. And that's why we have a D&D "multiverse"... because its the way Jeremy et. al. can confirm for every player the reality of their game's existence.
It's also helps explain why they are no longer concerned with trying to tie up every facet of every single story so that it all still "works" in some sort of overarching, complete D&D narrative "canon". No more lame attempts at retconning things to explain or justify that what came before so that it makes sense with what they are writing now. With the Multiverse theory they now don't
have to. Instead, every single one of us have been given the go-ahead to write D&D ourselves and have it be real-- just as real as anybody else's. So if someone says "You know... the firbolg race that they came up with for 5E, in the Forgotten Realms those are actually more Voadkyn that firbolg... WotC needs to explain why this got all messed up..." WotC can just say "In your FR world, maybe those
are Voadkyn... do whatever you feel is necessary and it is correct." And that's so they don't actually have to waste their time trying to write all that crap up themselves just to make some players happy.
There is no "canonical" D&D... because every single thing can and has changed. Both by the people who write the books, and by the people who play the game. And the D&D Multiverse allows for all of it to be true.