Out of curiosity, where did he say that? FWIW I ran the first half through Pangram (truncated to use free credits) and it said 73% AI, evenly interspersed. (I have no idea how much truncation breaks their algorithm.)
There is an "about" page on their blog where author discusses extensively (probably written by AI as well) about how most of his writing is AI-written.
I'm curious myself, in fact I was kind of relieved to read a long, well written article and just thought it felt human. I hope I'm right