Part of their demo is to sprinkle salt, baking soda, carpet powder, etc., into your carpet and have you vacuum with your old vacuum several times. Then, they will vacuum with some filter paper in the air exhaust of the Kirby to show how much crud your old vacuum is leaving behind.
With ALL vacuums, if you vacuum N times, you will pick up more dirt on the N+1 pass. You would get the exact same results if you ran the test with the Kirby first, then with your old vacuum! (Eventually, most of what you pick up is carpet material, not dirt.)