It should be obvious if current AI has been used to write any significant part on its own.

> The account claims to have analyzed several excerpts from the book using the AI-detector tool Pangram, which found the text was "100 per cent" AI.

And certainly nobody can blame them since Pangram advertises their service as near infallible

Related HN post about Pangram from a few months ago, particularly relevant to the "several excerpts" bit: https://news.ycombinator.com/item?id=49023243