He's correct - a stripped, never-before-assembled receiver is neither a rifle or a pistol or shotgun in the eyes of the ATF. That determination is made when it's first assembled.
If the receiver is assembled with a buttstock, it's a rifle.
If a stripped, never-before-assembled receiver is built into a pistol configuration (defined by ATF as no buttstock, and ONE grip at an angle to the bore), then it's a pistol, unless it's a smoothbore, in which case it basically becomes a shotgun.
The hard part with making a pistol from an AR-15 receiver these days is that is has to abide by the 'assault weapons' ban - which means that it has to be under 50oz, have no shroud or handguard, no threaded muzzle, etc - in order to retain the detachable magazine, because that magazine mounts outside the pistol grip.
It's VERY hard to make an AR pistol come in under 50oz.