AI is trained on human data. And high quality human data at it's best.

Can we assume everything we think as AI - must have had a high-quality human pattern behind it, and there is no way to 100% prove which is which - unless the author shows a screencast of them typing the artice?

This is not healthy. The right thing to do is – if someone doesn't like an article, they should ignore it – they shouldn't so confidently brand it AI without any proof at all, just because it fits their mood and style.