If I want to write a "historical" novel, everyone tells me there needs to be a purpose to setting a novel in that era.
I'm currently reading a book titled: "What I Saw, and How I Lied," by Judy Blundell. It's a National Book Award winner. It's about a young high school aged girl, set in the early 1950s. It's touted as a "historical" novel.
So, what makes it "historical" rather than "young adult?"
Yes, one of the characters is recently returned from WWII. The scenes and settings all depict how conditions were back then. But, in my opinion, it could as easily been set in 2009 with the soldier returning from Iraq or Afghanistan. So, what qualifies this book to be "historical?"
I haven't read the book you mentioned, so I can't speak directly to that one. However, as a general rule...
Historical fiction, simply defined, is when an author puts fictional characters against a setting of real historical events. Usually, to call it historical, the fictional characters need to interact with real historical figures or take part in/be impacted by the actual events.
What defines a historical event? Some people would call a novel set in the 70s historical fiction. Personally, I'm offended by that. I remember the 70s. Sort of. The 1950s is borderline. By the time you get to WWII, it would definitely be classified as historical.
Just because you can take a plot from one era and make it work just as well in another era doesn't mean it isn't a historical novel. However, the more the story depends upon the actual historical event, the more truly "historical" it is.