Even though Hat states that the key to the Fortress of Solitude weighs 500,000 kilos, it moves when he brushes the snow off of it.
In an obvious attempt to avoid interfering with the subplot regarding the loss of Brainy's Legion Flight Ring, the production team appears to have removed the Flight Ring that had been prominently displayed in the Fortress, with no explanation for its disappearance. If it were still there, Brainy could have made use of it.