He also said he'd taking it back out to experiment and going back to DI he noticed a definite decline in inaccuracy. So he put it back in. That is the other way around to what I would expect.
There are so many other factors that can affect accuracy besides just which gas system the rifle uses. There are plenty of piston rifles that are more accurate than many DI rifles; however, that's in spite of - not because of - the gas system.
If you managed to make two rifles that were identical in every way possible, except one had a piston and one was DI, the DI would be more accurate. The accuracy gap wouldn't be very large, and as gas systems improve that gap shrinks even more, but it's still there.