I wonder why Go and not Rust. Cloudflare iirc does use Rust for other infrastructure. I think Rust is probably the mainstream language with the best support for implementing high-performance/high-security software which cryptographic primitives certainly are. In addition, it is pretty easy to use Rust to write a library that can be callable from C allowing it to be used from multiple languages easily.

Go is a pretty good language in general but it does not have the raw performance that Rust/C/C++ have and doesn’t have the same support for correctness that say Rust does with its powerful type system. (For example affine types you that can help make state machines more robust to incorrect uses).