Some of those stories are non-fiction, like how Walt developed (or didn't) certain rides
ehhhhhhhhhh I'm sure a lot of that is fiction too.
I'm not a big fan of SEA, but a lot of that came from the implications of what a "society" of "explorers" traditionally meant. I think I can recognize that for the most part, the people that were putting in these references were doing so without any real malice or concern about how it would be perceived (although that too can be a problem).
But I am not against having backstories, a shared universe, or whatever other fan fictions you can think of. They've been around forever and some of them, like the volumes of backstory written about the Haunted Mansion and sea captains, have become an integral part of the fandom for some.