I get what King is saying. He’s excited about figuring out where his characters lead him. He finds his plot after the fact.
A lot of people take King’s comments, and similar comments from other writers, to mean that discovery writers just sit down in front of a blank screen or page and write whatever pops into their heads.
But it’s important — critical, even — to realize that the most successful discovery writers (like King) are ensuring that they hit certain beats at certain times, even when they’re flying blind. They might be making it up on the fly, but they’re still adhering to fundamental concepts of story structure as they do so.