It's not that surprising that this can be done anymore, with access to ProRes and a log profile, the ability to record externally and to control the phone remotely, it would be quite like shooting with a regular camera. Of course the sensor is much smaller, but that alone is not necessarily a problem if your workflow allows for that. Everything else can be worked around and in the end what you have is a modern sensor with few limitations.
Of course there's absolutely no good reason to shoot a movie with an iPhone unless you want to use it as a hidden camera or something. It's purely for marketing. When rigging it up like they did, using any normal camera, even something like a BlackMagic Micro Cinema Camera that lets you actually mount a lens would be much more practical. It's just so they can say it was shot on an iPhone.
Of course there's absolutely no good reason to shoot a movie with an iPhone unless you want to use it as a hidden camera or something. It's purely for marketing. When rigging it up like they did, using any normal camera, even something like a BlackMagic Micro Cinema Camera that lets you actually mount a lens would be much more practical. It's just so they can say it was shot on an iPhone.