Can't you just use a statically linked mold binary for the initial bootstrap, just like you use some kind of pre-existing, basic C compiler?

You could. But now you are trusting an entirely new toolchain that can build the Rust-based linker, even if it is statically linked. This toolchain is much larger than a comparatively small and much more easily understood C bootstrap toolchain.

This effectively triples or quadruples (maybe even more) the amount of code you need to trust for the cold bootstrap.