Rocker helps but is definately no replacement for setback. Rocker helps by increasing your dynamic nose height, this is the actual height that the nose sits if you were to draw an imaginary straight line from binding to binding out past the nose.
Now say we have 2 boards exactly the same but one is camber and one is rocker, you strap in and stand on the board on flat ground and measure the nose height. The rocker board will be only slightly higher because the camber board is being pressed flat negating the downward effect that the camber has. Lets say the nose on the camber board measured 50mm high and the rocker board measured 55mm due to upward effect the rocker has.
Now with both boards unloaded (similar to how is is when riding deep powder) and the imaginary line from binding to binding out past the nose again you will see that the nose rocker board has lifted up even higher, it is now at 60mm nose height while the camber board has taken a nose dive and its nose height has been reduced to 40mm above the imaginary line.
So the rocker board has a 20mm advantage in dynamic nose height over the camber board in true pow conditions. If back at the factory they were to increase the nose height of the camber board by 20mm both these boards would have the same dynamic nose height and would exhibit the same float characteristics in deep powder snow.