Is this "gun stealing technique" realistic?

It's a martial art technique to disarm your opponent.

From wideopenspaces, Watch martial arts expert Victor Marx execute the world’s fastest gun disarm:

Even wikihow explains how to do it.

Note: It's very risky move and in real life situation not everyone can execute it.

It is; however it's practicality is questionable. Gun disarms are taught foolishly by modern day martial arts/self defense studios. The problem is the people who're taught it, seldom practice it. And the people who teach it.......have probably NEVER had to use it in their life.
But they're largely impractical. It's overlooked by the rule of 3. If you're gonna hold someone at gunpoint, keep 3 feet away from them to keep them away from you. At three feet; that requires lunging at the armed assailant to even get your hands on the gun............A stupid thing to do.

A better strategy is to deflect the weapon's angle of approach by grabbing or push the barrel and attacking the opponents face/genitals. If they cant aim the weapon at you you're safer. Another safety tip, carry a fake wallet if you go out or if you live in a quesitonable area. Fill the wallet with two; one dollar bills sandwiched between some fake papers and some cardboard to simulate the thickness of cards. Someone tries to mug you give them the fake.