Originally Posted by Autonet
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.
It was a hidden message. His weakness (or krytonite) was the environment of his home world. Sure, it's part of the story, but the fact that it wasn't blatantly called kryptonite makes it a hidden message and an Easter egg.