It's old news for a lot of us. The ATF says a LOT of things that blow people's minds. AND, they change their minds from time to time, about what is, and isn't regulated, and how it is determined.
To find even a shred of logic (IF it actually exists) you have to look at how they define all the terms they use in definitions.
If it has a stock, and was designed to be fired from the shoulder, its a shotgun, a legal classification with a set of definitions. IF you cut the stock OFF, then its a "sawed off" shotgun (if it falls below the Fed overall min length)
If you cut the barrels short (below Fed min length) its also a "sawed off shotgun", even if the overall length is legal.
If the gun was never made with a stock, and was not designed to be fired from the shoulder, then it is a "smoothbore handgun", which is ALSO a regulated item, and has essentially the same requirements to own and penalties for non compliance that a sawed off shotgun has.
Its just different terms, and definitions all leading to the same place, if its not rifled, and under certain length specifications, then it is NFA regulated.
(also, just FYI, if it IS rifled, and under certain length specification, it is also NFA regulated - short barrel rifles and stocked pistols).