It's an odd way to cheat too, if they realised they had data from the validation set, couldn't they have over-trained a model with the validation set in the training data?
Yes, that would have been harder to detect if it is allowed. My understanding is that for a kernel competition (like this one), you can't use model weights that you've trained outside the kernel. Oddly, I can't find a rule explicitly prohibiting it.
Agreed, this actually happened in the buildings ASREA comp. the data was found online.