so no one has answered this and I dont know a good answer either, but maybe this will help . . .
A hall sensor detects a change in magnetic field, and can be used for encoders. Optical encoders will give a sharp square wave, but hall effect sensors will give a more rounded signal.
I am assuming that since they are using the sensor for the encoder it should be fine . . . because its two sensors, I am assuming its a quadrature encoder (detects motion both clockwise and counterclockwise).