I think it's a valid argument but I'd like to bring up some points of contention:
1. When was the last time Peyton Manning brought his team back from a deficit in the playoffs? I can remember one instance - the year the Colts won the Super Bowl and Manning led them back from an 18 point deficit in the AFC championship game vs the Pats.
2. Someone needs to address why Manning's teams always seem to fail at the worst times. The amount of times he's played on teams that have been considered contenders but get upset is a common narrative for his career.
2003: Colts finish the season 12-4. Destroy the Broncos in the Wild Card game 41-10 and defeat the Chiefs 38-31 in a game which there were no punts. Lose to the Pats 24-14.
2004: Manning's best season. 49 touchdowns, 121 passer rating in the regular season. Put up 49 points vs the Broncos in the Wild Card, 3 points vs the Pats in the Divisional.
2005: This year seems to be gift wrapped for them. They get a first round bye, the Pats get eliminated by the Broncos in the Wild Card. What could go wrong? Manning loses to the Steelers in the Divisional Game in one of the worst officiated games I've ever seen. The shoddy officiating is forgotten because of the Fumble by Bettis at the end and the game saving tackle by Roesthlisberger on Nick Harper followed by a choke by the Colts kicker. Manning played a terrible game though.
2006: Ironically, the one year in which they were not considered contenders due to their shoddy run defense. Manning ends up playing exactly one good game the entire playoffs though and his performance in the Super Bowl was average. He wins the big one finally.
2007: Colts get a first round bye, lose to the Chargers in the Divisional game. Can't really pin this on Manning; Rivers played out of his mind and the Chargers had some gutsy performances.
2008: Make it to the Super Bowl. Lose to the Saints. Manning famously throws a pick six when he attempts to make a 4th quarter comeback.
2009: Colts are not really considered a contender but they do lose in the playoffs to a team which they were favoured against.
2011: Missed 2010 due to injury. Comes back with a new team, leads them to a 13-3 season. Despite two special team touchdowns, the Broncos are unable to finish the Ravens in the divisional game. Manning throws an interception in overtime which eventually leads to a game winning field goal.