I want to learn this backhand shot

Not that difficult, just keep the ball short and low. His opponent gives him all the freedom he needs to do this kind of shots.

Second, he is pretty lost when his opponent manages to return the first spin. :)

Also you have to know that those edits leave out all the smacks that he missed. That's the problem with flat hits: really small margin of error. 5 mm lower is in the net. 5mm higher is out. Me likey bend ball with loop, so 5 mm lower or higher goes in either way.

Of course when flat hits land they are difficult to return, especially if they come from pips. Contrary to what someone above suggested, i prefer to play pippers and hitters with long balls. It MUST be long, or its not safe. Keeping it low is not possible with pippers, nobody has perfect touch vs pips all the time, not even ZJK. However keepin it deep is a decent insurance, because smacking a deep ball is just not safe, i don't care who you are. Smackers love spiny balls close to the net, where they can just CRUSH through the spin.