That's the exact promise. BotW has a main story, but it's not where the real fun is. The game devs will still write some overarching plot for people that want that, but you can just ignore that and live your own story.
There will also be a lot of fun in seeing how far you can stretch the original story to its breaking point, like how people enjoy breaking Skyrim with too many mods. Testing the limits is the fun. "What if I defeated Ganon by taming an army of crabs?"