I have also been an adult without children and one with child and I can honestly say that I had no clue as to how kids worked until I had one of my own.
I had many opinion including the fact that I thought parents could actually control their kids, that is, until I had one.
Some kids are naturally quiet and well behaved, some kids are just restless, some kids are off the walls, and some kids are restless, like the one we're talking about here.
From what I saw in the video, the kid was restless, but not that bad.
Most kids have tons of energy and it's very difficult to get them to sit still in any situation which requires them to stay that way for a fair amount of time and there's no controlling this.
As a parent, all you can do is try to keep them busy and distracted when they are forced to stop their energy filled bodies.
I was a ball of energy when I was a kid and I still am at 43
Unless the kid was totally out of control, the airline has a lot of explaining to do.