They were not. The primary stonework technique was almost certainly pecking [1], where you take a small "hammer" and whack it against the stone, pulverizing a bit at a time. This leaves a characteristic surface you can observe on the pillars today. However, the Gobekli Tepe builders were a bit smarter than just carving everything from scratch. They made use of the exposed limestone layers to quarry materials that were roughly the right shape and thickness.