Despite your scepticism, there are a few things that gave this one away.
Repeating the subject noun rather than changing to "it" after the first use is the biggest one in this example. It sounds very unnatural to keep saying "ChatGPT", especially three sentences in a row, but ChatGPT seems to do it quite often.
Having said that, I'm not confident I could do much better than 50/50 when not primed with the knowledge that people are likely to post generated content right now, plus the context of the conversation.