Because this is about allowing parts of the kernel itself to be written in rust.

This is for writing kernel modules, which ideally need to be integrated in the kernel build system.

Mostly this is surrounding (as others have mentioned) writing kernel components, such as drivers, in Rust.

Context on the post Linus says is: "If rust cannot report an error without aborting when it runs out of memory, then we can't use it". I have never written any rust code which does not abort when it runs out of memory, but I have never written for hardware (only with userland in mind) and my knowledge isn't that great anyway, so I'm not sure if it's possible or not.