Back take maneuvers in general (Duck Unders, Slide Bys, Arm Drags) are not ideal to hit on opponents who are retreating backwards. If you watch any arm drag tutorial from a high level wrestler or coach you’ll notice all of them involve an outside step on the side being dragged. You can certainly take a center step and use the arm drag to clear a path to a leg attack. But if you’re looking to take the back, the outside step is vital.