Earlier quoted context omitted.
I may be showing my age here, but I think most people would expect a TUI to provide arrow key navigation and highlight by default, which is not demonstrated. Simply mapping Gui -> TUI isn't really a good user experience IME. The terminal should generally be keyboard-first/designed with keyboard-only in mind.
Terminal.Gui maintainer here... We view the mouse as optional. We want the mouse to work perfectly, but it should be optional. If any of the library doesn't work well with just a keyboard, please submit an issue!
It could have helped showcasing that the UI works perfectly with keyboard.
I agree that the demo gif tends to say that this library is mostly intended for mouse usage.