There are some nuances here.
In order to meet the legal definition of a shotgun, the firearm must be designed to fire from the shoulder... in other words, have a stock (26 USC § 5845(d) ). A short-barreled shotgun, subject to the NFA, is any shotgun with a barrel less than 18", or any weapon made from a shotgun with an OAL of less than 26" *or* a barrel less than 18".
An NFA "any other weapon" is a weapon or device capable of being concealed on the person. The length typically quoted as being concealable is less than 26".
If you have a firearm that has never met the legal definition of a shotgun (ie, has never had a stock), then it can have whatever barrel length you'd like, so long as you are over the 26" threshold for an AOW. The Mossberg shockwave is one such firearm, the various pistol-grip-only "shotguns" are as well. Some places will sell you a virgin receiver that you can build out yourself.
Still, you cannot legally install a birds head grip and short barrel on a shotgun receiver without filing a Form 1.
So, depending on how he worded his response to you, he could have been right or wrong.