It isn’t cheating. A “mistake free” round is usually somewhere between -22 and -24, depending if there is a reachable par 4. That means reaching the green in 2 every hole and 1-putt every hole.
There are so many players in the tourneys, that a few hundred have those kind of rounds. Out of those top players, a few get lucky or really hot with chip ins, and you see the crazy scores.