It makes me nervous. Technically, he seems to be correct. For now. Have a look at Len Savage's letters. But I'm not brave enough to own such a shotgun.
I agree with the point that "capable of being concealed on a person" is readily reinterpreted. Chances are, if the BATFE caught someone with such a shotgun, that's exactly what they would do.
It would be very easy for a prosecuting attorney to demonstrate tucking such a shotgun into a jacket in front of a judge in order to demonstrate that such a shotgun is "capable of being concealed".