They're using QMK on their prototype keyboard and numpad input modules (which are RP2040-based internally).

The interface for input modules to the rest of the computer is "just" USB 2.0, so you could put just about anything there that'll fit in one of the input module form factors. Although high resolution, high frame rate touchscreens are probably out because of bandwidth constraints (since it is only USB 2.0).

It's plausible to use something like a DisplayLink controller, or for lower resolution even a microcontroller with flexible high speed peripherals like an RP2040 with the PIO interface.

Maybe you could snake a cable to one of the USB-C ports, if you really needed a display there. DIY foldable anyone?

Very exciting indeed. Thank you for pointing this out.

Indeed! They just keep doing good things (and doing them well from what I read)