I'm a bit lazy to look up for the source, but one article mentioned that the engineers were surprised that humans could get down to the 10 - 15 ms range by anticipating the light rather than reacting to it. They said this beats the mechanical buzzer, and it does happen, though not terribly often.

So yes, it is possible to beat Watson to the buzzer even if you both know the answer.

I suspect they could have improved the buzzer if they really wanted to, but it might have made things too lopsided.