AFAIK, Watson was just using the light signal to decide when to start buzzing in. Just the light, ignoring when Trebek was going to finish saying something.
Presumably a human could consistently beat Watson to the buzzer by predicting when the light would go on.
Not sure about 'consistently', though trying to predict the light seems a reasonable strategy to attempt.
There's a lock-out that applies if you buzz too early, and the variance of the 'beats' between Trebek's finish and the light, if that's a human-mediated process, may be far larger than Watson's light-fired reaction time.
Presumably a human could consistently beat Watson to the buzzer by predicting when the light would go on.