The people saying that were wrong, BTW -- Watson did have to physically press a button.
https://www.wired.com/2011/02/ibm-watson-speed/
> __Brown: __ Watson has a mechanical button-presser. It uses the same signaling device [the button] that the human competitors use in the game. Once Watson has decided that it wants to ring in because it has found an answer with a high-enough confidence, and it receives the signal that the buzzers are open and you can ring in, it then has to trigger the mechanical button presser and mechanically press the button.
Watson originally just sent an electronic signal. The physical button pushing was introduced later, to stave off the criticism that it had an unfair advantage.