I think posting someone's creation and calling it your own, even if it is tweaked a bit, still deserves at least a sentence or two of credit to the original designer. Now techniques are another matter, as those are not someone's "own" work, and can and should be shared throughout the Lego Community. Taking another person's credit for a creation is quite rude, and for a contest, illegal.
But since the original designer doesn't care, I don't see why you can't just email the guy who copied the creation to give credit where credit is due.