Supposedly people are raving about Swift being cross-platform nowadays, this seems like a simple example where the Swifties can prove how useful/practical that is in practice.

Swift is de-facto cross-platform without limitations.

SwiftUI is something entirely different and not trying to be cross-platform at all.

Ok, so Swift-the-language is cross-platform, but can't actually do cross-platform UIs. So great for CLIs, bad for everything needing a GUI?

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.