The iPhone Fold will come with one of two operating systems: iOS or iPadOS.
If it comes with iOS it's an iPhone.
If it comes with iPadOS it's a folding iPad, and Apple have some marketing to do to explain that.
Apple spent ages splitting iOS into iOS and iPadOS because they said the two devices do different things. If they now have an iPhone that uses iPadOS they've muddied the waters.
What I can see happening is the iPhone Fold will come with iOS, but iOS will have extra features enabled that allow it to work with the iPhone Fold's multiple screens/larger screen. You might not get fullscreen multitasking or even Stage Manager in the first release, but you will get apps that fill the screen, fullscreen movies etc. There will be compromises in the first version, as there always are.
At next year's WWDC in June, Apple will either:
- Preview the iPhone Fold and the new features in iOS, to give developers time to update their apps to make use of the new screen(s)/screen size. That would make it a success on launch, as Apple could market how "there are a thousand apps already ready for it," etc.
- Not preview it at all, and when the iPhone Fold is announced in September just before iOS 27 is released, apps will just magically fill the screen by default (if they're written well). A new beta of iOS 27 will drop which contains new features that developers can then use to update their apps. That would annoy developers though, as they would have to rush their updates, and there's a risk that showstopper bugs might've made it into the public release. Quite dangerous for Apple.
Just thinking out loud.