I know its a bump fire deviceHe is using a bump fire device. Bump fire is a really ugly work around to turn a semi auto rifle into a full auto. It really screws over accuracy in a big way.
The semi auto rifle sits in a spring loaded stock and the whole rifle moves from the recoil. The spring then pushes the rifle forward and "bumps" the trigger into your static finger, recoiling the rifle back again. Just look at how much the rifle moves compared to the stock. The principle behind bump fire would work with any semi auto rifle. And yes, you could build a device that would work for a Garand.
While not as accurate as a "normal assault rifle", its certainly accurate enough in trained hands. And it has no problem of spitting out lead at fully automatic speeds. An AR-15 is identical to an M-16 (which certainly is an assault rifle), except for some modifications of the firing mechanism (so it can only fire semi auto).