I will take a look, but right now I would be skeptical of any such "100% asm" "leak", considering how easy it is to get a lightly commented listing. My position is indefensible, so no point arguing though.

> C didn't start showing up in BIOS until the UEFI era (Tianocore etc.).

"Showing up" is an understatement, since Tianocore is almost all C. And we didn't switch from 100% asm to 99% c overnight.