The heading content and structure is the biggest tell IMO. Even shitty highschool kids don't write like that.
I don't understand where or how AI picked up that habit, because it's self evidently terrible. It makes it clear how low signal AI based writing is. The writing is like the music in shitty blockbusters; engineered to make you feel, rather than to actually structure the content or provide meaningful sections.
Compare this writeup to the Pixter writeup, where sections feel natural and not "scripted" like this.