There are many aspects that go into intelligence, a riddle is nowhere near enough to categorize somebody's intelligence overall. One can be bad at riddles in particular and still be quite smart.
The intelligence aspect of riddles is often the abstract thinking required, most often not taking words literally and instead searching for alternatives. For example legs could be referencing far more than human or even animal appendages for that matter. Perhaps the legs of a tripod, or something so absurd as counting wheels as legs since they serve the same function on a vehicle. I've run through these things and don't happen to have a match with this particular riddle, I may be overcomplicating it. Or maybe not
