Yeah, the standard applies to every game, but the standard sucks. Yes, odds are someone with 45 game winning drives is far more likely to have more clutch last minute game winners than someone with 25, but that isn't guaranteed.
But if you look at Pickett, he is a well below average QB who has a defense that keeps opposing offenses under 20 points in most games. Other than the Browns game this year (which the Steelers won on a defensive TD at the end of the game 26-22), Pickett has yet to win a game in his career where the opposing team scored more than 18 points.
I don't think he is clutch. I just think the Steelers win tight low scoring games where the Steelers just happen to score last.