Complexity optimization is a generally useful skill.

"moving the goal posts" is called "every month at work, when you finish 1 task and then move on to another task, instead of going home and getting paid to rest on your laurels"

moving goal posts, is "I asked you for x, you provided me with x, but I really wanted y, so now I'm going to tell you you're wrong"

You want someone to answer a programming question within a particular set of boundaries, you set those boundaries.

I don't think it has to be "wrong". If I were asking this question, and I was provided with "store them in a hash", I would say something along the lines of "that's a correct solution" and then add additional constraints.

"How you adapt in the face of additional constraints" is very much relevant to programming.