Yes, it's true that the app could hand-track "recoil", just like it's seemingly tracking his gestures in the 3D GMail.
I still think it's staged. The motions look way too "casual" and not deliberate enough to be interpreted easily in software without some revolutionary tech, in addition to the unrelated revolutionary tech needed to do the AR "live".
The video was created by Weta Workshop who specializes in special effects. "Providing design and manufacturing for film, television and creative industries for over 20 years."
I would not put much stock (no pun intended) into this unless there are more videos like the first one in the article mentioned.
The gun appears to be a real, physical device. Given that there must be a camera looking forward in order to interpret not only the scenery but recognize gestures, it's highly likely that what triggers the shot isn't the subtle pressing of a trigger (unless there's some kind of bluetooth connection here) but rather it's the "fake" recoil motion that the system recognizes as a gesture in order to trigger the shot.
Thus, the shooter is recoiling the gun himself, to fit the narrative.