As someone said just because it's magic doesn't mean it automatically affects him. It will compel him to tell the truth, and it will bind him. But it's not magically enchanted to act stronger so that Superman's body weight would break his own neck or even squeeze his windpipe to suffocate him. The rope being magic doesn't mean it automatically takes away Superman's powers. Superman is still super strong and invulnerable the magic in the rope doesn't affect that.