What's with the purism? It's just a term used to differentiate one way of making a UI from another. Who cares about what is practical when you're just trying to give a thing a name.

Even in your example, it's pretty clear cut. If the window is built with text and served in a terminal emulator, it's a TUI. If you build it with a graphical framework that now needs X11 or whatever, it's a GUI.

This is just needlessly pedantic.