It's a very unusual way to use the flaps.
Did he maintain a constant power setting during the approach, or was the throttle at idle?
Is he also a glider pilot? He seems to be using the flaps as a glider pilot would use airbrakes.
Perhaps he selected flaps 30, and then changed his mind for some reason unconnected with height, (Perhaps because the crosswind was stronger than he thought) and just appeared to be controlling the height with flap?
You could always ask him!
In any case, I see no danger in adjusting the flaps from 30 to 20 at 400'-500'.
MJ
