That is kind of doubtful. The video clearly shows a small puff of smoke presumably above the ejection port when he fires the gun for the first time. The Welrod and its modern descendants have a bolt which is locked in position when the gun is fired and it has to be cycled manually. Thus no gas would escape the gun at the ejection port when the gun is fired.

It is more likely that this is a semi-automatic pistol which will not cycle properly for one reason or another and the shooter has to manually complete the cycle by racking the slide.