Interesting, but I doubt they are doing it to lose height, as they are still flapping which indicates they are using energy to maintain or gain height. Why would they keep flapping if they wanted to descend? They'd just go to high AoA, dangle their legs, feet outstretched for max drag, and perhaps roll 30 degrees or so side to side.
They would certainly stop flapping!
I find it quite common that ornithologists just don't understnd even the most basic principles of flight!