Of course, the impressive thing is that Watson can answer the questions at all.

The point being that at the highest levels of play, all of the contestants know most of the answers. This is a very important thing to remember. It means that even in all-human games, it's not "do you know the answer" but "can you ring in fast enough."

So it still is really quite impressive that reaction time matters at all to Watson, because it implies that it can compete at the highest levels.