I think the D turning it on...has a lot to do with with your above examples. Although Flacco and Manning were unbeatable in those years because they didn't turn the ball over in the post season.
No doubt Flacco got hot in the 2012 playoffs. That defense was only average, so they needed the QB to play out of his mind. That might be the best 4 game stretch of Flacco's entire career. That 4 game performance lead to him becoming the highest paid QB that following season, which ended up being a horrible choice by the Ravens as he never played to that level again.
For Peyton the story is very different. In the 3 playoff games in 2015 he threw for 539 yards, had 2 total TDs, 1 Int and lost 2 fumbles. Peyton was pretty much useless that year. He hurt (only suited up 10 games), had more picks than TDs (17 to 9) and had the lowest yards per game (224), QB Rating (68) and QBR (44) of his entire career. I'm a big Peyton Manning fan and I remember watching him that year and it seems as if his throwing velocity dropped so much that he didn't know if guys were open any more (because he couldn't execute anymore). Unlike Flacco he didn't turn his Superbowl win into an extension, but rather retired.