Quite frankly I think no one would legitimately take them up on it. You would have to make it apply only to existing, released games as it would lower the likelihood of actually costing Epic any money (from the loss of revenue as few to none would actually do it) and it's a lot easier to prove that a released/finished game is already using the Unity.
But the positive response from just offering it would be a good PR move, IMO.
But the positive response from just offering it would be a good PR move, IMO.