I've seen knock sensors like that before, too (the kind you can just use your hand with).
I guess I don't really understand the point behind this new one. You need to carry the little box around with you and enter a pin-like number into it. Then you hold it against the door while it knocks.
I would expect that short-range RF would be better (either a transmitter or RFID). This would also have the advantage of no exposed mechanisms. I guess someone could read the RF, but with a pin and rolling codes, this shouldn't be too much of a problem. You ought to be able to monitor the knocking with a sensitive microphone or other pickup, so I don't imagine that it would be that much more secure.