I've seen the text, and it was genre typical.

You get 80% of the way there just by giving them a writing sample and asking them to copy the style paragraph by paragraph. You get a lot further with some basic statistics to look for seriously out of distribution word use.

The hard problem isn't prose, but consistency over a long text.

I don't know how he's achieved that, but I've seen the reviews and none complained about consistency issues, which readers tend to be sticklers about.