The author is a professional writer, so depending on what specific writing tasks they're doing, they may be able to do portions of their job almost automatically, devoting far less than full attention to their work. (I know a few writers who do this.) Additionally the movie is silent, so I'd imagine that leaves the language parts of the brain fairly free when watching the movie.

That’s true. Although now that I read the article, the quoted sentence comes right after they mentioned 9 hours of a stationary ship so I guess it’s possible to keep that on in the background and not miss the plot.