I agree with you that it's much more interesting than the language, but I don't think it matters for a project like the Kernel that already had its build system sorted out. (Especially since no matter how nice and convenient Zig makes cross-compilation if you start a project from scratch, even in Rust thanks to cargo-zigbuild, it would require a lot of efforts to migrate the Linux build system to Zig, only to realize it doesn't support all the needs of the kernel at the start).