> But he is now telling his C devs they will need to learn and code rust to make new code in the kernel.

I don't think this is accurate, Rust is still totally optional. Also, the Rust folks are supposed to fix Rust code whenever it breaks due to changes on the C side. If they fail to do this, the newly-broken Rust code is supposed to be excluded from the build - up to and including not building any Rust at all.