If you look closely at what the navigationBarTitle call resolves to, it isn’t the method you linked:
func navigationBarTitle(_ title: Text, displayMode: NavigationBarItem.TitleDisplayMode) -> some View
It is:
func navigationBarTitle<S>(_ title: S, displayMode: NavigationBarItem.TitleDisplayMode) -> some View where S : StringProtocol
It can’t resolve to the first one because you are not passing a Text as the title, are you? You declared the argument title to be a String.
Here’s the documentation for the second method. From the documentation we can clearly see that it is available from iOS 14.0 to 14.2.
The reason why using a string literal as the title works (such as in ContentView) is because you are calling yet another overload of navigationBarTitle:
func navigationBarTitle(_ titleKey: LocalizedStringKey, displayMode: NavigationBarItem.TitleDisplayMode) -> some View
LocalizedStringKey conforms to ExpressibleByStringLiteral, so string literals can be passed to an argument of type LocalizedStringKey, but not a variable like title.
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