It doesn't matter if Green Goblin was on the battlefield at the time the cards were discarded
Fortunately, thats why we have Kangaskhan or Hypno, and those decks have significant weaknesses that Mewtwo can exploit: Charizard EX takes a long time to set up without Moltres (which Mewtwo EX can OHKO) and Blastoise EX requires a lucky Misty draw to reliably get its Energy
But then it was "printed" at Common in a Coldsnap theme deck, even though there was only 1 copy per theme deck, so he was exactly as "rare" as the rares in that deck
Yet, HAT has an ironic twist: the banning of Heavy Storm lets trap-heavy strategies flourish, with Artifacts and Traptrix unleashing some of the most diabolical disruption in the game, shutting down plays with ruthless precision
As I was not involved in Magic 2015 , I can only guess at the evolution of this design