Interestingly I just skimmed through a video linked in TFA: https://www.youtube.com/watch?v=SlGRN8jh2RI and Boris Cherny explains that they chose TypeScript and React for Claude Code primarily because it was "on distribution" and the models back then just weren't good enough at other languages.

So it's interesting that Codex is written in Rust. Amongst other things it could mean OpenAI had more powerful models that could handle Rust, or their engineers had to handhold the agents a lot more up front, or Rust has structural advantages that could overcome being less represented in the training data.