> But a human developer, even a not so great one, might know something about the characteristics of the actual data a particular program is expected to encounter that is more efficient than this AI-coded hybrid sort for this particular application.

Sure. But then that belongs in a test case that 1) documents the assumptions, 2) demonstrates if a specialised solution actually improves on the naive implementation, and 3) will catch regressions if/when those assumptions no longer holds.

In my experience in that specific field is that odds are the human are likely making incorrect assumptions, very occasionally are not, and having a proper test harness to benchmark this is essential to validate the assumptions whether or not the human or an AI does the implementation (and not least in case the characteristics of the data end up changing over time)