My neighbours regularly remind me that AI is quite different in French - despite being Francophone they tend to use it in English for what they think are better results.
It's entirely plausible detection is totally different in different languages.
On how much text? There's a fair few reports that it's quite inaccurate when given smaller samples of text, and if you go cherry-picking the odds will likely go up even further.
It's in the article - looks like they ran it on a book.
No, they say they ran "passages" of the book through it.