That video is a bit off. The curve as weight decreases scales rather than shifts to the left by the square root of the ratio of the actual weight over the maximum weight. This is because the dynamic pressure, and therefore the resulting lift at a particular airspeed and AoA, varies as the square of the airspeed. Cut the weight in half and, for a given AoA, the plane will need 70.7% of the original speed (sqrt(1/2)) to develop matching lift.
Not sure about this case as I don't know how much water was on board, but if it was 1000# of water and max wgt of 2900#, then the factor for reaching stall AoA for a particular load factor will be scaled left by 0.8, so the maneuvering speed, by that calculation should be 80% of the fully loaded case.
I feel like I am missing why the maximum load factor doesn't go up as the aircraft is lighter. Sure, by 8 or 9 Gs the pilot is the limiting participant, but design for fatigue life results in higher attachment capacities. It also doesn't seem clear why where the payload is stored makes no difference to the maximum load factor. Putting that container on a wingtip would certainly change the characteristics in a way the simple calc fails to capture.