I don't see much wrong, or how this would be cheating. They produced a winning entry, it should be on the organizer to ensure that their test data set isn't trivially findable. It would be like testing a digit recognizer on the MNIST data set and being surprised when someone just hashes it. A real solution isn't to force opensourcing it is to get better metrics. Maybe add a random component like a GAN to generate potential test data, and see if anything classifies that correctly. In the real world when the metric becomes the target it ceases to be a good metric. So test what you want to test and not just some existing data set.

Edit: I didn't see that the test data was given. See the first reply to this comment.