The difficult part here is getting something through Apple's blockades.

The crazy part is that, out of the box, the adapter runs unix: https://hackaday.com/2019/07/30/apple-lightning-video-adapto...

If I can run Doom on a dongle or on an Ikea wifi lamp, why cannot we get a portable computer in a smartwatch?

I suspect bloated complex operating systems won't make this happen.

i don't get it

Doom is running by using the hardware (CPU, RAM and I/O) which is embedded into the HDMI dongle.

It is not using any of the computing resources of the Mac.