They've stated multiple times that they don't rehearse, don't have a script.
Personally I see no reason to disbelieve them. They are quite used to improv and of course they're talented actors. I assume some general issues and potential story arcs or changes for individual characters may be discussed offline, it's something I do with my own players. But in my experience it's typically a player saying something like they're thinking of multi-classing so we discuss the basic direction to take and then we'll work it into the story (like when
Sam wanted to take a break from Scanlan and then brought the character back). But that's far different from discussing what's going to happen before the production.
I guess believe them or not but every DM has to find what works for them and what doesn't. Trying to copy another DM exactly is generally not a good idea.