The video is sensible. most of the time the traders sentiment is against the market trend. (and why its in this way refers to the philosophy of traders minds and herd behavior. which is not really the subject of this thread.)
In short as I remember it was around 11 or 12 years ago that a retail forex broker (fxcm) start to release its clients position on the net called "Speculative Sentiment Index (SSI)". SSI has been released two times a day, short after fxcm some brokers released their clients position cumulative long and short ratio. (including: Oanda, Dukascopy bank, Alpari And Easyforex which later changed to easymarket ) after some years some of other brokers released the ratio too.
on the first days of SSI some traders had been started to analyzing the data. those days it was something unique. between mentioned brokers only fxcm oprator one time told that the SSI is a contrarian Indicator. while other mentioned brokers never explain more and stated that "these data are only the clients long/short ratio and we dont have any more explain".
after analyzing the long/short ratio by those traders and by another group later, these conclusion I remember have been made:
1- Most of the times the crowed is on the wrong side of the market/trend.(based on the long/short ratio.
2- on a long choppy market the Ratio is less reliable.
3- The Ratio is more reliable on FX majors.
4- Its better to use the average of position ratio of several brokers data not just one broker.
5- The Ratio has nothing to do with scalpers,
6-the best usage of long/short ratio is for swing trading strategies which involved longer duration.
above is the short story of Long/short Ratio as far as I remember.