Originally Posted by dnno1
An Easter egg is in intentional hidden message in a story or a film. The Kryptonian environment was Superman's weakness (or kryptonite) which was a subtlety (i.e. a hidden message or Easter egg).
That's not an easter egg, it was part of the story. If anything it was a tangential callback to Kryptonite, or a newer take on the idea around it being a weakness.