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.
People who shout about ancient aliens and hidden technology always completely ignore that the people in the past had access to the most advanced machinery available: humans.
Add time to that, so that your project can have a scope of generations, passing down knowledge gained from a lifetime of doing this particular thing, the results become unsurprising even.
Amazing, inspiring and ingenious but also unsurprising.
[1] https://stonetoolsmuseum.com/technique/pecking/