I would rephrase this as “the solution can be as simple as your model of the problem but no simpler.” A better model, structure, implementation, or restatement of the problem could transform the dimensions of the problem space radically.

But I do agree with you in spirit of what you said, that there is some essential “hardness” of the problem that is part-and-parcel with the difficulty in stating or explaining the problem. I see a similar complexity in representing the problem in code or functions or algorithms etc. Then comes a related issue with reducibility; some concepts or problems cannot be broken down into smaller discrete parts. They are already as simple as they can be. This is ideal in a certain sense, in that the concepts can be explored with fewer assumptions. However when someone asks you to explain like they’re 5 years old, interpretation and perception starts to creep in through intuitions and abstractions.