Pretty much all of this game was written on the run lmao. He has key points, but he improvises how he gets to those. It's a normal way of writing and I think most writers do it this way, is just a fun way to do it.
Making it up as you go is a terrible way to write, you should have the major plot points planned from the start. Mysteries should only be mysteries to the audience, the writer should already know the answer. I hate that JJ Abrams style of writing, creating "mystery boxes" without bothering to put anything in them.