The book’s rule concerns nafs/ʿayn applied to attached nominative subject pronouns.
جِئْتُ أَنَا نَفْسِي: I myself came; anā is detached pronoun and nafsī is semantic emphasis.
جِئْنَا نَحْنُ أَنْفُسُنَا: we ourselves came; naḥnu is detached pronoun.
These are additional teaching examples.