I’m not saying that a flying theatre isn’t possible (practically anything is at this point), but there are certain pieces of evidence that lead me against one.
Firstly, I believe that the building shape doesn’t quite line up with a flying theatre. The hipped roof part is more pronounced than that of a typical flying theatre; Flight of the Sky Lion only has a very small slope in its roof, whereas this one looks quite considerable. The wall edges also aren’t quite the same; Flight of the Sky Lion has three angled edges at the end and is shaped more like a pentagon, whereas Horizon looks like it has one straight edge at the end and is shaped more like a rectangle.
Secondly, the building ground space doesn’t really line up with a flying theatre either. The dimensions Alton have given us suggest that the ride portion of Horizon alone will take up a minimum plot of 3,500m2 or so, possibly even exceeding 4,000m2. Flight of the Sky Lion, taking up a plot of less than 1,000m2, suggests that that is far too big for just one flying theatre, and Voletarium at EP, taking up a bit over 2,000m2 for its ride portion, suggests that it may even be a tad too big for two flying theatres. Anything more than two screens would be unlikely at Alton Towers; even Flight of Passage at Disney World only has two screens, and Epcot only added a third screen to Soarin’ retrospectively. I would also counter that if it had two screens, surely the roof would likely be hipped on both sides rather than just the one?
Thirdly, the height doesn’t quite line up with a flying theatre either. Flight of the Sky Lion’s building was described in its planning application as being 13m in height, while Alton Towers have described Horizon’s building as being approximately 20m in height. I don’t see why Merlin would add an extra 7m onto the building height unnecessarily, particularly given the existing controversy surrounding that site’s utilisation in the first place.
It could well be a flying theatre, or some new variant of one. But I’m not currently convinced… I could be wrong, but the evidence I see before me pushes me towards indoor coaster.