Zig's safety is lower than Rust's, that's true, but it is still a very big improvement over C, so a case could be made for it in principle. But of course in practice Zig is not stable enough to consider that yet.
Anyhow, this isn't all or nothing: a case could also be made one way or the other about how much 'unsafe' to allow in Rust in the kernel. It's a question of tradeoffs.
The reason Rust is interesting in the space is the memory safety without a GC. Zig doesn't have that.