So their promotional pages proclaim. Considering how noisy the G4s are, this really isn't hard to do, especially as the phrase "3 times quieter" is not qualified with any real measurements. It could mean almost anything in the world of marketingspeak.
Still, they appear to have developed a case design superior to the standard PC tower ATX: "The Power Mac G5¬ís enclosure houses four discrete thermal zones to compartmentalize the primary heat-producing components. Fans in the zones spin at very low speeds resulting in a system three times quieter than the Power Mac G4."