The point is to shorten the minimal bootstrap path to Rust.

With your suggestion you can't use rust until after you already have rust-to-wasm transpiler available (which almost certainly itself already requires rust, so you are back where you started).

The article mentions that the Bootstrappable Builds folks don't allow pre-generated code in their processes, they always have to build or bootstrap it from the real source.

The generated C code could contain a backdoor. Generated C is not really auditable so there would be no way to tell that the code is compromised.