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?
I can't see the video from here, but assuming the bird intends to lose height, it won't need to flap its wings. It only needs to hold them extended and - though I don't know avian anatomy well - certainly has muscles to enable it to do this, though they won't be as strong as the muscles powering the downstroke. After all, if you watch birds at rest, they move their wings upwards to preen. Hummingbirds almost certainly have an active upstroke as well as downstroke, and humans have flexors and extensors and abductors and adductor muscles for all the relevant axes.
But... Interesting point. Perhaps there is some more subtle reason for the maneuver.