To be honest I thought AI was already used to flood popular message boards with user content trying to sound natural to show user engagement.

I think, part of the "problem" is that most of their training data is formal language. They'll have much less data from private chats and such.

You can write in the prompt to the AI to write in hip and casual tone, but you will get something that will give you "How Do You Do, Fellow Kids?" meme vibes.

I haven't really tried but I think if you were trying to use LLM generated text deceptively the model could probably do exactly as you describe already so long as you prompt it to do so clearly enough.