I think they want to make the distinction that SwiftUI is not necessarily to replace Storyboards, although it will replace them.
UIKit works okay in code. But unless you have experienced people actively laying groundwork, it's IMO more likely to be a mess than SwiftUI. Even the explicitly declarative part, Autolayout, will only be understood by like 10% of the team and the rest are kinda winging it. Using Autolayout outside of Storyboards makes it less declarative, so it is then more conducive to programmer error (like non-idempotent updates).
UIKit works okay in code. But unless you have experienced people actively laying groundwork, it's IMO more likely to be a mess than SwiftUI. Even the explicitly declarative part, Autolayout, will only be understood by like 10% of the team and the rest are kinda winging it. Using Autolayout outside of Storyboards makes it less declarative, so it is then more conducive to programmer error (like non-idempotent updates).