I don’t really understand this objection. For every tool that I use, am I supposed to divine the best underlying language for it and then determine whether or not it is written in that language? Don’t I have better things to do?

The only assumption you need to make is how the process went about, which was described by Jarred on a HN comment when the PR was first discussed: they had prompt that described exactly how things should be translated, for each "pattern" they were using in Zig, an appropriate equivalent was described in Rust. Zig and Rust are not that different, both are system languages and things can be done similarly in both languages, so architecture-wise I would think the exact same thing would work fine. I am not sure whether the LLM actually wrote a transpiler which just followed the rules, or if it did the job itself, since that information is not public yet, as far as I know, but my guess is that the LLM wrote a transpiler to do the job, then reviewed/fixed compilation issues, then fixed tests. And I'm pretty sure some human interaction was part of that as well.

We do have an idea, and it contradicts your guess: https://news.ycombinator.com/item?id=48133806