As another commenter noted, there’s no Rust code currently in the Linux kernel. There may be some soon, but not much - it’ll be a dominantly C codebase for a very, very long time, maybe forever.

As for seL4, doesn’t that just support my point that highly secure software can be written in any language, even C? seL4 is primarily implemented in C. There “executable spec” is written in Haskell, but the actual kernel that people use is written primarily in C and a little Assembly.