Baker wings don't create a flat bottom (at least not on an ACC). They sit slightly above the plane of the bottom surface of the chassis. Keith describes the effect as being similar to a V-bottomed boat hull. The idea being that it makes it easier to level the rifle when compared to a wide, flat plate, while still providing lots of surface area for stability (and grip tape covered bottom surfaces for friction). Since the wings are attached to the chassis, they also keep the rifle's bore axis closer to the surface of the bag than a plate that clamps to the bottom of the chassis.
But the Area419 plate is removable, and you can attach the bag directly to it.