He’s described Fuzix in the following terms:
Fuzix is a proper Unix, with multiple processes, devices, all the standard Unix tools, Bourne shell, vi clone, etc. Given the RP2040’s limited RAM and no MMU, this port keeps just one process in memory at a time, swapping in and out to an SD card to do context switches; this works surprisingly well. Each process gets up to 64kB of code and data.
He succeeded in his efforts, and he describes performance on the little board as “pretty decent”, though he hasn’t tried overclocking the Pico yet to see how much he can get out of it. He has documented it all over on his blog where you can see how he did it and how to reproduce it yourself. Over on the Raspberry Pi Forums, where he posted the work as a thread, you can see the exploits of other people who have replicated the project, as well as some more information on hooking up an SD card reader like so:
David’s plan is to look at porting Fuzix to the full Raspberry Pi at some stage, so keep an eye on his blog for that.