People looked through both in-game (how we found things like the lati@s mega stones) and through the pgl site (how we found things like the discount coupon and gems) and there are no sort of key items for events so far as we can tell.
I will say that the reporter is probably referring to, like, Diancie or something. Mythical Pokemon refer to event Pokemon.
But otherwise I agree that these "mysteries" are not even really mysteries. They're just things for flavor that don't have any ~secret meaning~ that absolutely no one, not even any of the guides, have mentioned.