The memoization capability only disappears in the very last step, which I would argue is not the meat of the post. Up to that point, very valuable refactorings

The memoization capability only disappears in the very last step, which I would argue is not the meat of the post. Up to that point, very valuable refactorings and simplications have been applied. One could very well have replaced the final transformation with a factoring out of the algorithm into a free function, which the constructor merely calls and caches the result of, without meaningfully changing the point of the post. (In fact, that would call out the memoization capability as an orthogonal aspect even more strongly, since you could reuse the same construction for memoizing other free functions.)