FWIW I work on AI and I also trust Pangram quite a lot (though exclusively on long-form text spanning at least 4 or more paragraphs). I'm pretty sure the book is heavily AI written.
How long were the extracts you gave to Pangram? Pangram only has the stated very high accuracy for long-form text covering at least a handful of paragraphs. When I ran this book, I used an entire chapter.
I ran the introduction chapter through Pangram [1], which is one of the most reliable AI-generated text classifiers out there [2] (with a benchmarked accuracy of 99.85% over long-form text), and it gives high confidence for it having been AI-generated. It's also very intuitively obvious if you play a lot with LLMs.
I have no problem at all reading AI-generated content if it's good, but I don't appreciate dishonesty.