I agree, but his example is strong. While calculating something every time the mouse moves would be expensive and memoization is probably a good idea, the mouse pointer in most situations has some locality, and so some LRU cache is probably a better approach. There's probably examples where you'll get more bang for your buck using a different cache storage as well.

Does that mean in general their memoization function is flawed because it doesn't support that, no. If you need such specialization, for optimization, you're free to roll your own--which is why I agreed with you.