Yes, I'm sure they're extremely smart, but equally, they are deeply expert in C, and it is not that easy to become deeply expert in Rust (or any other language). Even smart people can't just pick up new tools in a day.
They way rust is used in the kernel is a lot more constrained in scope and APIs compared to normal userland rust and hence easier to learn. A lot of added rust API are also just high-level abstraction of already existing concepts/code used in the kernel.