I mean, we're probably not going to ask someone to solve a brand new problem as part of a take-home project. We're intentionally not trying to be tricky, but to cover well-known territory so there's plenty of prior art and examples to fall back on. As another case, if you were hiring as a frontend dev, we might ask you to write a simple interactive web page using vanilla JavaScript and not jquery or such. It's awesome that you know React! Great, you'll fit right in! But first, can you use the stuff built into the browser without loading an extra library? Or put another way, are you capable of extending web frameworks if the need arises and not just using them?

You're kind of right about it being the XY problem, except it's where the misdirected candidate is trying to solve "how do I do thing X" when the real problem is "write a simple program in Y so we can know if you know Y". We aim to be very explicit about the project's requirements so that none of this is a surprise afterward.