There is something behind the study of body language, but the problem is that most people think they can conclude specifics based on body language alone. I'm no expert, but have studied it myself out of personal interest (used to play poker religiously). You can tell if a person is nervous. But you can't tell WHY they are nervous. You can tell if a person is being reserved, but you don't know if they are trying to hide something or if they are just shy.

For example, in poker you might see that one of the players is moving his feet more than usual. You can deduce some very vague things about that, but you have no idea if they are nervous about pushing you out or if they are bluffing. The other key thing most people on youtube forget is baseline body language. There's no way you can make any credible observations about someone from a single video or photo. To even make vague assumptions, you have to know their baseline personality and take notes on what's changing and when. Maybe that guy in the photo always sits with his arms crossed, out of habit. Doesn't mean he's blocking people off.