The only way this could possibly work would be to send the entire text of every book into ChatGPT. If you ask ChatGPT something like “does the following passage contain depictions of sex: [text goes here]” it might work. However, ChatGPT’s context size is smaller than your average book, so you’d need to break it up into multiple requests.
Even with ChatGPT this is probably still not feasible to run against an entire library of books, so I would guess they’re half-assing it.
Technical feasibility aside, this is a stupid, evil idea. I hope everyone involved in this scheme gets fired.