And if "can't go pro until 3 years have passed" was an NCAA rule, I'd guess it would result in a lawsuit and the NCAA would get their hands slapped HARD. A player like Smith could just declare for the draft and dare the NCAA to find some way to enforce it, and then sue, and win.
But it's an NFL rule. Which means he can't just declare for the draft and sue the NFL if they won't accept his declaration and allow him into the draft, because if he loses the lawsuit that could impact his NCAA eligibility. Which he'd probably have to sue over, and then could be left out in the cold for two years if he loses that one too.