What is an AR15 pistol? The picture in Ammoland shows a rifle.
Reporter is likely an ignoramus - more likely the “pistol” in question was a pistol grip on said rifle.
AR15 style rifle with a shorter than 16” barrel and no butt stock (originally ;^). The advent of the “arm brace” made them a lot more useful. The arm brace is intended to fit over your forearm while shooting the pistol, but it works against your shoulder also.
Basically a way to have a sort of short barreled rifle w/o the Federal registration and $200 tax. A true butt stock is nicer, but then the $200 and registration...