I suppose he means that a small microphone/speaker setup near an ATM machine's keypad could allow you to track the position of the finger and get the correspond

I suppose he means that a small microphone/speaker setup near an ATM machine's keypad could allow you to track the position of the finger and get the corresponding keypresses without any intrusive modification to the ATM itself. Of course that would only be valid if you proceed to then steal the person's card right afterward, so all in all, not that useful.