That was the point of the article, it's not easy at all. You can't just write Ruby in Rust literally unless you want untyped bags of key value pairs and virtual functions everywhere. The languages have different fundamental semantics and there are many non trivial decisions, including and up to, total architecture change.

For example, in ruby some kind of stringly typed reflection utility might Dry out code. In Rust, that might mean writing a trait over some subset of types and a macro helper, but would require careful dosing work.

Letting the LLM decide is the approach in question and the author was pointing out how spectacularly it failed to translate a complete, human designed system.