Flaps 15
As I recall, the Flaps 15 selection results in a lot of "aft" movement of the rather complex and sophistocated flap assembly, and not a lot of "down" extension. This setting of flaps adds surface area to the wing more than it changes the overall shape of the total (wing plus flap) airfoil. Whatever removed the lowermost flap panel on the starboard side probably did not drastically change the amount of surface area, right wing compared to left wing.
Others here have speculated, and I agree, that the 3# engine does not seem to be producing as much thrust as the other two, based on the "fly off into the sunset" photo previously posted. So, the combination of less drag from the missing starboard flap panel was likely offset by the (presumably) reduced thrust coming from #3, while not resulting in a large reduction of lift from the right wing. Had the amount of lift produced by the right wing been significantly reduced, they would have never gotten airborbe. They were probably very close to Vmc as they became airborne. Maybe this is why they were able to remain airborn long enough to bring her around for an approach.