Another thought - this is somewhat more tenuous, in a regulatory sense, than the bumpstocks. It seems to me that a revisit to the centerfire/lands-and-grooves question could make this a "rifle" PDQ.
To me, the lands and grooves are the... wait for it... rifling that make a rifle a rifle. That they are straight just means that it is sucky rifling. Has the actual ATF letter been released yet?
Something tells me they are relying on a pretty specific definition of "rifling"...not without precedent:
U.S. v. Meadows, 91 F.3d 851, 856 (7th Cir. 1996), emphasis added.The term “rifled” is derived from the verb “to rifle,” which means “to cut spiral grooves into the bore of (as a firearm or piece of ordnance).” Webster's Third New International Dictionary 1954 (1986). 8
FN8 The grooves of a rifled bore cause the bullet or projectile to rotate and thus increase the accuracy of the shot.