It's definitely a nice video and an interesting phenomenon, but I doubt whether the commentary is accurate.
Upside down wings generate more drag causing a goose to slow down quickly
Although I agree with the first part (upside down wings generate more drag), I doubt whether geese (or birds in general) are able to properly fly, or float, upside down. In the sense that they fly level and their bodies experience minus 1G.
Birds are "high wing" creatures for a reason: Muscles can only generate tension, not compression. So they need to be underneath the wing, just like the strut on a C172. At the other end they are attached to the breast bone which is comparatively huge, compared to non-flying animals.
Now turn the bird upside down so that it flies inverted properly. Where are the muscles and the attachment points that are able to sustain these forces?
Personally I think what they're doing is a sort of barrel roll, but from level flight. This allows them to rapidly lose height without picking up a lot of speed. In the video you can see that the bird in the upper right corner of the frame loses height rapidly, in what seems to me a more or less ballistic flight path. So they're not properly flying inverted (-1G). As a result of that, you cannot claim that their wings are inverted, and that that creates a lot of drag.
Nevertheless, very nice video. Shame it ends so quickly.