The benchmark is the interesting part. Models get no source code and no web access — only the ability to run the target program and observe input/output. A full reimplementation means devising the whole structure yourself, not translating code piece by piece.
This is a companion discussion topic for the original entry at https://www.reddit.com/r/LocalLLM/comments/1v8doue/mirrorcode_opus_47_reimplemented_a_program_from/