Say what you will about Rust vs Zig as languages, the Zig toolchain is definitely the easier of the two to integrate into another project.
First they essentially wrote a translator that preserved the C idioms (so it wasn't idiomatic go) until they had byte for byte output.
Then they started changing code one by one to be more Go-based.
Bun is doing the same. Right now it is mostly a one for one translation of the zig code. Over time they'll make it more idiomatic Rust.