The differences would surely be more noticeable - at least potentially - if they weren't using such a distorted metal tone....
Of course the player influences the sound, but the "....theory that the player and not the equipment is the single most influential part of a guitar sound" is patent nonsense. How about another experiment - one player playing the same riff through four completely different rigs, whether it's different guitars (maybe get a Tele or a Ric in there), different amps or both? I'm going to go out on a limb here and say I'm pretty sure I'd be able to tell the difference.