Yes, on many of those shots he is to some degree. I think you're misinterpreting what I'm saying. I'm not saying that they're falling backwards and are off balance. What I'm saying is that they're shifting their weight forwards towards the ball, then once they come to contact they accelerate their racket head as fast as possible. In order to do this, they pull their arm back and to the left. Depending on how much spin vs pace they want to put on the ball and their forward momentum at contact, they'll move back to a different degree. Sometimes it shifts their weight, sometimes it doesn't.
This is a great example. Fed is finishing into the court, but he's still pulling back and to the left to get maximum racket head acceleration.