This position-specific application of a theory is what I find most irritating about debates concerning the greatest quarterback of all time. Yes, quarterbacks impact the outcome of a game more than any single position. However, when a quarterback as supremely talented as Dan Marino only has the opportunity to play in one Super Bowl, let alone win it, and far less talented quarterbacks, such as Trent Dilfer, Doug Williams and Jim McMahon lead teams to Super Bowl victories, you would think that pundits would realize the need for a more nuanced analysis or at least apply the same ludicrous theory to all positions.
With respect to fourth-quarter comebacks, there are a couple of points that I don't believe have been mentioned previously in this thread. It is entirely possible that good to great quarterbacks on dominant defensive teams may not have the opportunity to pull off as many fourth quarter comebacks, simply because they don't surrender as many points to begin with and they are less prone to surrender fourth-quarter leads due to their more consistently high level of play. Conversely, great quarterbacks who are frequently saddled with mediocre to weak defenses, such as Marino and Manning, are more frequently put in the position to orchestrate fourth-quarter comebacks, due to their defenses' inability to securely shut the door on opponents.