I agree with Fosse76 on this one. From an artistic standpoint, I think calling it a "movie" is a bit misleading, and not really in line with the terminology we usually use around filmed stage productions. However, most filmed stage productions don't get a theatrical wide-release, so from a business/marketing standpoint, I can see why they would use the word that most accurately describes the context in which it's being distributed.
I wonder if they will go all the way with this branding, and submit it for Oscar consideration. Has anything like that ever happened before? The closest thing I can think of are some of the old-school Shakespeare films that were adapted very closely from specific stage productions, and still, in many ways, feel like a filmed stage production. Olivier's "Othello" is the first that came to mind, having been nominated for several Oscars. But again, this example isn't actually equivalent to this current situation. It's just the closest precedent I can think of.