That, plus the quoted text basically says the model homed in on the monomyth (Hero's journey) structure; while the pattern was identified and named by a 20th century American writer, the pattern itself is common and as ancient as it gets. Wouldn't really call it anglo-american bias.

The monomyth is also writing 101 these days, and considered the default structure you can and should use if you have little experience writing stories, so naturally it'll be a high-probability result of an LLM prompted to write a story - especially prompted in a way that implies the user is inexperienced at writing and needs a result suitable for an inexperienced writer.