Or it could be a ATF agent that doesn't understand how regulations and laws have been enforced in the past. You can't assume the ATF is correct given their history of inconsistent actions and flip-flops in the past.
My understanding is that in the past the ATF ruled that the bore for for a Destructive Device is measured across the top of the lands, not across the bottom of the grooves. The rifling for a 28 gauge is .498 across the tops of the lands. That's less than the magic half inch.
The only way we'll know is to wait and see what comes out eventually.