dav1d is the av1 decoder and it’s an insane feat of engineering. Written in assembly, it even eschews the normal c calling convention to get even better performance.

The normal C calling convention is really only for cross-binary calls (e.g. between shared libraries). If you're not doing that you can ignore it; it's not a weird thing to do. It would be odd to strictly follow it in assembly and I assume compilers don't either.