Comment by ssivark

The string comparison model is useful in the regime where string lengths are much shorter than list lengths and can be bounded by a constant. It would be a horribly broken (i.e. unrepresentative) model if you were constructing intermediate strings whose lengths could grow arbitrarily and become much longer than list lengths.

One can of course make a puzzle out of anything, but it behooves asking what situation is being modelled by that problem.

And yes, this criticism might apply more broadly to TCS rather than just this specific artifact from OpenAI. It's reward hacking regardless of whether humans do it or LLMs.