It's a misconception that the AR is a direct impingement system. If you compare the gas system of the AR to the direct impingement used in the Ljungman rifle, you'll see there are significant differences. In the original patent got the gas system used in the AR, it makes it clear it's not direct impingement.

Another misconception is that somehow the AR doesn't have a piston. The fact is, the AR does have a piston. The tail of the bolt is a piston and where it fits inside the carrier is a cylinder.