SwiftUI could not have be written for the public version of Swift. Instead, the Swift language has been changed to have SwiftUI's desired syntax. That level of symbiosis is not "just" a UI framework.
Swift is first and foremost Apple's own language for Apple's own frameworks. The frameworks aren't layered on top of an immutable language. The language is evolved to serve the frameworks' needs. This has already started with Objective-C 2.0, but with Swift it's even more obvious.
>SwiftUI could not have be written for the public version of Swift. Instead, the Swift language has been changed to have SwiftUI's desired syntax. That level of symbiosis is not "just" a UI framework.
Yes, but that's neither here nor there. The changes in Swift to make SwiftUI easier can be used by others to implement their own (Windows, Linux, whatever) Swift UI frameworks as well.
Obviously Apple, as the core devs of Swift and sole devs of SwiftUI would have more influence on Swift direction.
It's an open project, but not some "community designed" language. On that regard, not very different from C#, Java, etc.
Objective-C was already like that since NeXT exists, stuff like protocols did not exist originally in the version that NeXT licensed from Brad Cox's employer.
Swift is first and foremost Apple's own language for Apple's own frameworks. The frameworks aren't layered on top of an immutable language. The language is evolved to serve the frameworks' needs. This has already started with Objective-C 2.0, but with Swift it's even more obvious.