"S" turns were taught to me in early stages of hang glider training. It is used as inexperience to judging height when too high on final. Most I saw used on first few flights then used a more standard approach. From the ground it really looks like an inexperienced pilot, but the judgement for approach was right on for those that I saw using it. I think there were a lot of hang glider pilots that did/do not have any other type of flight training and so no good judgement of height, and I think the tendency of new hang glider solo would tend to be high on final.

I have never been taught "S" turns in glider training. I would much rather track slightly away on downwind, and or extend, than to have to "S" turn on final.

If that video is from the 40's, I wonder when this was stopped being taught..