That's the Rys Mk II demonstrated in that video, isn't it? Then there must be a mistake in the specs as it clearly says support for USB/HID and USB/PS2 keyboards
While using the ST mouse/joystick port makes sense for those wanting to use a USB mouse or joystick it's not of much help to those of us who are into replacing the existing ST keyboard with a USB type.
But using the internal (on the ST motherboard) keyboard connector should allow for this, as already discussed using a Raspberry Pi Zero board. But literally being a computer on its own I would like to avoid that route.