I guess you could say that.
There are frameworks such as swift-cross-ui, but I couldn’t tell you anything about their maturity.
I don’t think CLI and GUI applications are the only software people write. Swift is a great backend language.
To some degree. There are _many_ SwiftUI clones that support other frameworks such as Gtk and Windows, with varying states of maturity. Or you can share the business logic and write the UI natively in Swift.