You are absolutely right. But when someone from New Zealand is tweeting as an American or someone from Manchester tweeting as a Russian journalist, a subset of fake news can be flagged. (eta: I guess what I am trying to say is that we first have to identify the "unreliable narrator" archetype.) For example, we have AP stylebook in the United states that is related to associated press. Every publication has a recommended style book. Now...can it be mimicked? Yes, of course. But we have at least have one level of authentication. It takes years to master stylebook rules for a human. A fake news generator might be able to pass the Turing test, but I don't think its been done yet.