there is a number of different methods. One is a separate model/classifier trained with human annotation. Here's a paper by OAI: https://arxiv.org/abs/2305.20050
that's a bold statement without any supporting argument. more like an opinion really. specially trained classifiers can be very good. sentiment analysis has been a thing for a while now, there had been entire businesses built on this tech where being even slightly wrong can be very costly
"very good" is the exact same level advertised as the base models they're "protecting". It's not good enough, otherwise we wouldn't be having this conversation in the first place.