Have you compiled and run it?

Not yet - it uses Qt6, so I'll wait until Devuan 5 (Daedalus) is released for that.

(Also, possibly until I can find time to try wrap my head around why the CMake/Ninja build process is so convoluted.)

It'll be interesting to see if/how the serenity thing evolves, but for now its stated goal, "a love letter to '90s user interfaces with a custom Unix-like core" appears to be mainly DE -- and (I think) is a solved problem, e.g. XFCE (which is my goto), LXDE, and some others -- unless the "custom core" (not a Linux kernel?) will be doing something different. I like that it is ~100MB though. That is different!

I'm less interested in the OS than the browser - though it may be interesting to see where it goes, and reassurring to have another backup in the event of Linux and BSD OSes becoming unworkable - but yeah, one of the points of Serenity seems to be about being [mostly] self-contained, so it has its own kernel, but also its own and window manager/server/etc.

On one hand they may be spending time re-inventing the wheel, but at the same time there's merit in having a consistent self-contained system with understandable code.

(I had a look at Serenity's code for "mkdir" command, there's no stupid comments there, and it is concise and comprehensible despite me not being a C/C++ developer. The same is not true for other mkdir implementations.)