I've seen LLMs produce terrible code indeed, but I have also seen humans produce terrible code. I haven't dug in to JS runtimes specifically but have read plenty of code in openjdk and cpython - there are many points that could be done better, but there's also no point since it's working, and keeping working code unchanged tends to be a smart decision in software engineering.
So of course the last point brings up whether it was a good idea to rewrite bun if it was working. Apparently the bun team thought the difficulty in getting changes in zig upstream meant it is. I don't intend to hold LLM code to a higher bar than human code - notably if the runtime continues to work, that is as good as I can expect from what is otherwise a huge black box of extreme programming (not that agile kind).