Within the context of the stories, Superman's powers, as great as they might be, are the result of a natural phenomenon, not supernatural, so he's affected by most forms of sorcery in the same way as any mortal.
Superman's powers are the result of a natural phenomenon. This is within the context of the stories. This is not supernatural. This is as great as they might be. He's affected by most forms of sorcery in the same way as any mortal. This was within the context of the stories.