I tend to agree with you, and @snelly, and @chrisecurtis.
If you Google ‘major motion picture iPhone’ you will find several references to movies and/or scenes that have been filmed on iPhones. Martin Scorsese used an iPhone for one scene in Wolf of Wall Street, and Steven Soderbergh has filmed at least two entire movies using iPhones (7 & 8).
My experience is mainly in still photography, and IMO lens quality and lighting are the most important factors in getting a good image. Another is size. The camera has to be able to fit in the needed location. That’s one reason GoPros have probably been used in many more movies including The Martian, Furious 7, and The King’s Man.
Choosing the right camera is like choosing the right computer or tablet.