Well, they use a linux kernel which is written in C and some assembly. All of the overlying components are apparently written in rust. These components still interact with the system calls exposed by the kernel.
An example of a kernel itself written in rust would be the Redox microkernel. It seems to be able to run a graphical environment but it hasn't been updated in a while.
Did you even read the blog post? This is running Linux, and only the AWS software-components running on it are largely written in Rust. It is still Linux and the OS is not written in Rust, nor the container daemon and probably none of the non-AWS software packages.
That honestly sounds terrible, and apparently only works on ECS. What a shame. I was thinking that AWS was actually going to provide something useful to the open source community.
but these guys have built an OS in rust.