From the youtube video it looks like he has the color sensor and ultrasonic sensor steered by the same servo. When he first starts it you can see it turn the color sensor all the way to the right. My guess is he does that to establish the angle. He turns it, resets the angle to zero, then knows how many degrees is straight forward. After that it starts sweeping left and right, and the car drives with left, centered, or right steering.
"The more you know, the more you know, the less you know."