You are very correct. I have edited the response for this move to:
"1...Nxd4 is a good attempt as the Knight is immune from capture after 2. Qxd4 Rh5+! winning for black. However, if you take a second look there is a better move which will force a speedy checkmate."
http://www.chess.com/chessmentor/view_lesson.html?id=135
I was doing this lesson and played 1. ... Nxd4. I got the following response...
1...Nxd4 is based on an hallucination. Black thinks that 2.Qxd4 (drawing the Queen away from the defense of g3) is impossible due to 2...Qe1+ but overlooks that 3.Qf2 is possible.
Click 'TRY AGAIN' to go back and make a different move.
... However, after 2. Qxd4 doesn't 2. ... Rh5+!! win immediately for black?