As seemant pointed out on Discord, there is something that is not quite allowing the new post-indy kits like gnome-kit to autogen, as pipewire is not showing up autogenned in gnome-kit:
https://code.funtoo.org/bitbucket/projects/CORE/repos/gnome-kit-sources/commits/fa59880ebc8974532fe721d4d33a581ec3dfd564
This was part of FL-9362.
I'll investigate.
FL-9362 mpv, pipewire build issues