Simply that: did the events of Hunter x Hunter: Phantom Rouge really happen (with respect to HxH story, of course)? At least the timeline is credible (between Yorknew City and Greed Island arcs).
It's a part-canon side-story.
The premise of the movie is based off a manuscript of the Yorknew City arc written by Togashi Yoshihiro that he eventually ended up scrapping.
The film is based on an unpublished story manga creator Yoshihiro Togashi wrote around 10 years ago.
Since it's based off his own work, Phantom Rouge presumably has Togashi's blessings.