I really think that it was a cool hack by William Walter Wu. When I first saw it I was expecting it to use Leap Motion[1], which made into HN's first page a few days ago[2].
Getting Leap Motion integrated with Chrome would be amazing, because, you are right, the webcam alone does not seem to be precise enough, as the author said:
It took me a while to write and fine tune the detection algorithms. Even then, the algorithms are only about 80% accurate
the accuracy does not seem to have reached the level where it would be really usable yet. 80% accuracy means every 5th slide the transition goes wrong. but i suspect pouring some machine learning magic dust on this could get it there.
* multiple directional scrolling * hijacking my back button
At least they're trying. I would think you'd need something more sensitive than a normal webcam for gesture detection, so that's a feat in itself.