The runtime is written by people, not machines.

The compilers and low-level toolchain libraries that encode machine instructions have been machine-translated.

The runtime is an example of very good low-level Go code that breaks safety rules. However, it is not a good example of user-level Go code. Note that even though the Go code in the runtime heavily uses unsafe, the runtime is much safer and more stable (we find less bugs) then when it was written in C.

The main thing is, that they machine translated the Go compiler itself from C to Go, so the compiler had not so great Go code. The rest of the runtime probably was already human-written in Go before that. And of course, anything which has been under development since then is human written, like the new SSA-based compiler.