Don’t remember, but I could swear I read it and then I saw the penalty called a couple of weeks ago. I no longer have a copy of the book, alas.
The rule on punts is that the kicking team can’t have its players go out of bounds while covering a kick unless the player is blocked out of bounds and tries to get back in bounds right away.
The rule is designed to keep players on kick coverage from doing things like running all over the field. The receiving team can have its players run pretty much everywhere on punts.