I saw someone on the old indoor range last weekend using that exact same CAA kit, and wondered the same thing.
Rules are usually made for the stupid not the smart. So even if we know that the CAA is just a chassis for the Glock using the same barrel and doesn't affect the backstop any differently than a stock Glock, some moron might see it and think it's cool to break out their Hi Point carbine, AR, or shotgun.
Almost every time I run into Zoo he tells me some story of how someone got caught trying to shoot buckshot or a non-22lr rifle on the new indoor range, so I could totally see them adhering to a blanket rule that even the stupid can understand.