In the real world, when you make one move, your opponent will respond with one move. To assume that your opponent will do nothing (as showing in that clip) when you "parry" is not realistic.
If you can make 2 moves while your opponent can only make 1 move, that mean you are twice as fast as your opponent. If that's the case, anything you do will work.
That is good logic (and a common flaw in non competitive styles), but there is more than that.
Regardless the clip, that I did not watch, trained people will not react to a 'Inofensive' jab, which becomes something else In the way. And you did 2 moves when the opponent is starting the first move. If the opponent overreacts your first move, it becomes even easier. Of course, trained people will recognise that 'Inofensive' jab second time it comes...
There is psychology, or whatever, up to physics.