No, I have not asked the ATF for a clarification on the gas piston versus the carrier extension.
It does not matter either way to me since I change enough parts to U.S. that I don't need to change one of these.
Technically speaking though, a piston is not a "GAS" piston unless it's under direct gas pressure impingment. The carrier extension see's no direct gas impingment, thus it is merely a piston, not a "gas" piston.
You can have all sorts of pistons, mechanical, hydraulic, knumatic, etc, but each see's direct impingment from some other form of energy. In the case of the Saiga, that energy is gas from the barrel.
My guess is that the ATF never disassembled the Saiga, so they were probably unaware that there was a seperate gas piston in the cylinder and just assumed the carrier extension has hit with direct gas impingment like any other AK.
Of courese the ATF could rule either way, that's why I don't rely on that particular part as one in my U.S. "count".