The forend covers the barrel for shielding of heat and gives the shooter something to hold on to while firing.
That would be a very dangerous statement in court, because it would acknowledge use of a second grip on the firearm, so if you had been charged with having an illegal AOW you acknowledging it as second grip would be bad.
Arguably it is only a rail, intended as a way to attach perfectly legal accessories like lights, lasers etc.
However what is interesting is firearms where it is not a rail, but is instead an actual horizontal grip are legaly sold as regular pistols.
Firearms such as the Auto-Ordinance Thompson pistol:
The Encore/Contender pistols:
The Ruger Charger (10/22 pistol):
On and on it goes.
Notice the bipod on the Charger would even itself make an excellent vertical grip if you closed the legs together.
So is it any clearer?
These firearms could not be sold in some states under state law if they were AOWs.
To further complicate matters federal law varies from one Federal district to another based on case law and precedent which is different in every district, unless it is a specific issue that has been address by the US Supreme Court.
So what is considered what can be different in different portions of the nation. Some districts have precedent that can help win a favorable decision, and some have precedent that can be used against you, and others have no precedent of thier own on some subjects.
One district may support the view something is an illegaly held AOW, while another may feel it is a legal handgun.
The Supreme Court of the US picks and chooses which cases it takes on a voluntary basis, so they can never even make it to them.
The ATF operate in all of those districts.
Further even when a favorable decision is made by the Supreme Court of the United States, the ATF then undoes thier decision, by saying it only covered something even narrower than it did. As in the Contender case (regarding being able to legaly convert a pistol into a rifle and back swapping barrels, and having a stock while in rifle format), which clearly settled some important issues, which the ATF has since said are not settled, and still have thier own interpretation that conflicts (but not according to them.)