Usually, people are talking about an active noise canceling property when they bring up noise canceling. Maybe I'm making the wrong assumption here, but I'd assume that the author of this is not referring to the fact that all closed headphones will cancel noise.
My mistake for missing the fact that they actually did make a headphone that has an output... that just seemed like a silly idea. I'm a little disappointed to discover it's real. The addition of other headphones would just cause the sound to become progressively worse, would it not?