Is it not the case that the ratio has to be at least 4.5g?
The actual indicated airspeed would have to be 4.5 times the unaccelerated stall speed in order to pull 21G before stalling. So the unaccelerated stall speed would have to be 100 mph. At any stall speed less than 100, it could pull more than 21G at 450 mph IAS. (not accounting for energy lost before reaching 21G)
Edited to add that if the unaccelerated stall speed is
greater than 100 mph, the 21G is a stretch. And 21G
seems like more than the trim change with loss of tab would produce, but that's pure conjecture. Maybe if there is enough good video to measure the angular change of momentum over time, the load factor can be calculated that way.