How is this NOT a Short Barreled Rifle?
My first thought as well.
Because that is not a stock, that is a stabilizing arm brace. Apparently this video was shot during the period when the ATF warned that shouldering a stabilizing arm brace constituted a 'redesign' of the part and would result in an SBR. Note that the shooter in the video has a cheek weld but the brace does not touch his shoulder.
The ATF has since rescinded that opinion letter.
With the "stabilizing brace" answer already provided, a second question is why with a Republican President and GOP control of Congress are we still beholden to NFA rules?
C'mon Pubbies! Throw us gun-owners a bone already! We sent you to Washington! Repeal this crap!!!
Notice how much the back of the gun jumps up and down in the video? It isn’t firmly against his shoulder. This isn’t bad form, it is a legal requirement. If you use the ‘arm brace’ as a stock then you could be in trouble.