Such "tests" are really deceiving for the common viewer, because one cannot guarantee repeating test conditions. It's a quite random occurrence for the mud/sand/gunk/whatever to enter the critical areas of the tested pistol - most of the times it will, but many times it will not. We see a mud-covered pistol, but what we don't see is if that mud made it to the locking lug/s for instance. It's true that some guns by design handle heavy fouling better than others, but I can guarantee you that once mud, or sand enters the barrel locking lug area, firing mechanism, slide rails, or firing pin/striker channel, the gun will stop functioning. Out of those the barrel locking lug/s is most vulnerable because it is readily exposed, opens for mud to enter at every shot and holds a tight tolerance, that is very, very sensitive to debris. Close next are the slide rails, but apart from tolerances, the recoil spring weight is of much importance there. I can bet that if Tim repeats the same test over and over again with the same gun, he will get different results every time.
P.S. And he did test that 1911 with cocked hammer, that although correct ("correct" from a 21st century CCW point of view), did put that pistol in immediate disadvantage as it allowed for dirt to readily enter the insides of the pistol.