Certain patterns are much more common in LLM output than in human writing. I'm a journalist and love an em dash, for example, but I've never met/read another journalist that uses them nearly as often as LLMs. Same with the "this isn't just X, it's Y" pattern. When you have multiple of these patterns in every paragraph, it's a pretty clear indicator that the text is AI-generated.

Plus, the author admitted to using AI to write it.