Almost every pistol is used with two hands which results in "two-handed fire"...pretty much since forever.The issue, the definition of grip is not clear, but what is clear is that it "indicates two handed fire". So can a handguard be a grip if it indicates two handed fire? I think a jury of randomly selected peers would be out on that one
AR and AK pistols have been around for a long long time and have never been an issue and probably never will be. But a secondary grip on a pistol has been pretty well defined by the aft and a handguard isn't it.
Like I said, go look it up on their site on the pistol rules...not on that SBR vs Pistol worksheet. Call them. They'll tell you over the phone nine times out of ten.