Looks a more than decent answers to me.
Do you think anyone can use this G-OpenAI platform to write posts in bitcointalk forum, and is there a way to detect this?
Maybe we need to have some kind of detector for text written by bots, but I often think that some people are worst than AI after I read their posts

For sure this can happen.
And, as far as I can see from the various example I see around, ti might well already be the case.
If you search for that, you'll see dozen of AI- based services selling AI generated blog posts, Longform, twitter thread and so on.