This isn't about GPT, but specifically about the moderation endpoint OpenAI provides (and also uses internally). I'm not sure if they published how it works, so for all we know it might not be a large language model but something much simpler. It's also free, so both in terms of cost and added latency there are good reasons for OpenAI to do something more clever than asking GPT3.5 "is the following offensive".
But apart from that technicality, I think asking a sentence to be treated as equally offensive no matter if it uses the word "men" or "women" is very reasonable and doesn't justify slippery slope arguments.