I think we are actually saying the same thing! We could think of situations where the cache would be verboten (sensitive info), or where it would be mandatory, like in your example, or optional like in my example.

My aim was to voice disagreement with the "code is spec" crowd, whom I think are using a different (and in my opinion tautological / useless / counterproductive) definition of spec. Probably because they are mad that I use trigger words like Vibe and Maxxing, and they assume I can't even read the code I'm shipping. I digress.

In your "time complexity is a downstream requirement" example, which is a great one, I think you would prefer to have well-maintained written documentation of that criterion that lives outside of the procedural code itself, would you not? How much attention that doc gets is a matter of process and preference, but I'm advocating it should get more (spec-first).