Hulk is unlocked in the game by collecting Gamma Regulators. Basically like the action figures from MUA1. You get 5, you unlock Hulk. Thor and Jean Grey are also unlocked via their own “artifacts”. The code you get will let you unlock him without collecting anything.
They said that they wanted to avoid overpowered / mystical characters in the “core” roster of the game, so they made the mythical or overpowered characters unlockable in a unique way so. They wanted you to feel like you need to “summon” those characters, rather than them just being there.