The signs of body language are NOT language. They are an expressive graded communication.
If you go out on a cruise, for example, with a person whose body is extremely tense, you might infer from this sign, a lack of confidence in the ship.
On the other hand, if the boat was in danger of capsizing and your friend was lounging lazily in the passageway, you might infer from this sign, a lack of concern despite the situation.
Although, these signs are important, they are NOT examples of language.
It's a wonderful and important book, but ironically, it is NOT a book about language. It is a book about signs, emotional graded signs to be exact.
Graded in the sense that messages are conveyed by a degree of body function a change of voice pitch or intensity, speed of movement, degree of distance, or body tension, etc.
It is expressive because it involves emotional issues.