It's a shell that _doesn't run in a terminal emulator_ so comparing it to maybe the oldest terminal emulator around is missing the whole point...
The point was if I run "xterm &" I get another window with another shell, and if I run "open" I get another window with music, an image, or a video, and it works both locally and with a remote X connection.
The real big thing here is not the shell, it's the display manager that permits it. This is also what permits a window manager that can be controlled from a shell.
Window managers and display servers are a dime a dozen in Linux, you can pick and choose whichever you want.
So a custom shell couldn't clear the screen, open borderless windows, fill them with text/images/video/whatever, and send events to them to control them based on commands the user types unless the machine is also running Durden and Arcan? Maybe I'm missing something but I have to say that seems like a bold statement.
Also if this new shell throws out the terminal emulator and depends on a display manager then how does this work remotely?
Finally there are also terminal emulators like Kitty that have taken a different approach, but still offer similar functionality to Cat9, using X11.