That isn't the problem. AI assistants have limited memory, and they see the chat through a window. AI cannot scroll back through the chat, so it cannot remember the earlier parts as the chat progresses. There is also a permanent saved memory function, but it has its own limits. The AI assistant simply wouldn't be able to remember the base plot as it continues.
That's probably why the YouTuber was able to tell many novels are written by GPT? Because of tons of inconsistencies that are impossible if written by humans. And also the novels might not be generated by GPT, just written, if that makes sense. You just put together a really rough draft and tell GPT "yo rewrite this for me" and it does it.
All the AI books written by the cheap AIs have the same awful style and cadance. It's the premium service that I worry about. I'm not sure how many new fictions are being penned by AI, because I'm not looking for them, but I suspect it's fairly high.
Ahh when you read more and notice the same writing voice (or lack thereof) you tend to pick up the patterns you don't see many quirks or unique features in writing styles that often anymore...