I don't think the parent is saying that the professor annotated the book, as that probably doesn't make any sense. As I interpreted their story, the professor

I don't think the parent is saying that the professor annotated the book, as that probably doesn't make any sense. As I interpreted their story, the professor had a habit of asking questions and then answering them himself, if nobody offered an answer. Students wrote down those answers in the margins of the book. Over time, the book collected a lot of these notes from different students.