Unfortunately, the comments in your version of DifferentialPilot are out of date. In all the DifferentialPilot methods, the sign of the angle parameter determines whether the direction angle of the robot increases (positive) or decreases as it moves. Imagine the robot is at the origin facing along the X axis. The sign of the radius determines whether the center of the turning circle is on the positive or negative Y axis. So if the radius is positive and the angle is negative, the robot backs up so its direction angle decreases. And if the radius is negative, and the angle is positive, the robot again backs up.
Sorry about the confusion. I think the latest version has the correct documentation.