You are not listening! Equal illumination of both sensors does not stop the motor! The circuit is too simplistic and impractical. It needs TWO comparators set up with a dead zone in the middle (where the two sensors have equal illumination) where the motor is stopped. This means that the circuit has THREE states: CW, OFF, CCW.
Your posted circuit only has ONE comparator, which can output only TWO states: CW and CCW. That means the motor is ALWAYS running, back and forth, back and forth,....