This is categorizing with a pretty broad brush, it seems. I took a look at a random day, and the articles that are claimed to be AI are Waymo publishing a model for self driving cars and a post about scams in Apple News ads.

Sure, Waymo’s model is AI and the scams in Apple News are often AI generated, but this isn't some sort of monoculture of LLM articles.

Ironically, this article could have been improved by using an LLM to categorize things and also publishing the prompt, so we could inspect what it means to “be AI”.

The article already states that distinction. It doesn’t say that everything is written by AI, the heatmap even has a legend with different colors for “article about ai” and “written by ai”