Here's the thing about these sorts of demos; watch the other guy.
If someone throws a jab, leaves it extended, doesn't move their feet and turtles on the counter...yes, what this Sifu is doing would be effective. Good luck finding anyone that would do these things. More often than not that punch is followed by another, and another, and another, and suddenly that flaily arms extended WC attack he is doing is a prescription for getting knocked out.
When they’re are sparring more heavily he’s not leaving the jab extended.