I think he was referring to the Syrac being adjustable from a fully assembled upper with handguard installed. The Seekins needs to be adjusted prior to handguard installation. With the Seekins you will have to take your rifle out to the range without the handguard installed to tune the amount of gas. Or install the handguard, shoot it, remove handguard, adjust gas, reinstall handguard and repeat if necessary until you get your gas setting dialed in. If you shoot lighter or heavier loads, repeat above process. If all you do is shoot one type of load, the Seekins is still a good option.
With the Syrac, you can adjust the gas settings from an installed handguard. In the gen2 model, you can count the number of detent clicks from fully closed for each type of load used. Nice if you're shooting subsonics one minute then switch over to supersonics.