They define "Work" vaguely because they're lawyers. Yes, the legal status of the data can depend on how you produced it. No, it's not a problem. Don't first write your book in Apple's app and all your imaginary problems go away. Do anyway and export it as text, your imaginary problem is still there but still imaginary; Apple won't bother you. They just don't want you to sell software produced with their tool on other platforms.