Sorry to tell you but it’s not their problem. They have no responsibility in this matter it’s basically a verbal contract made by the developer than no one can enforce
I Can´t believe that someone can make a contest and then just disappear. I really preffer to think this was a mistake and he forgot to check the forum (?) ...
No sympathy from me. Most people enter these competitions and leave a fake 5 star review. Makes a mockery of the App Store reviews If the dev had disappeared at least you have a ‘great’ 5 star app eh ?