Fizban's is just the latest in a long line of books that changes the facets of 'canon' in the games. It has changed over and over across these past 40 years, and in 20 years time, that concept of 'canon' will be changed again as well.
So why anyone actually worries about what is * currently * "canon" is beyond me. Right now there is just ONE "platonic ideal" of Demogorgon, and anytime any of us deal with him in just an echo? Well, that wasn't how it used to be, and come 6E when the new wave of designers change everything up to make the new game different, it probably won't be that way anymore either.
If people would just stop thinking about D&D as what is printed "right now" as being the only thing "real", you'd all probably be a lot happier. "I don't like how they have made Spelljammer in the new book." Well, then don't use it. Use your old Spelljammer material. It is just as useful and true than anything printed today, because pretty soon the stuff printed today will be seen as just as "unofficial" as 2E Spelljammer is to so many of you all apparently right now.
There is no universal D&D truth. Because the truth always changes. All we have is what happens at our table. Treat that as true and go hang everything else.