People consider Manning a "choker", because in the weeks leading up to the Super Bowl, they were calling him the G.O.A.T., just like someone else has already said, forgetting the fact that entering this playoff year, Manning was only 7-8 in the playoffs, with 4 of those wins coming in their Super Bowl victory year, and that this was the only playoff year in which he actually played great.
However, I don't think you can call someone a "choker" based off of 1 play, and while he did throw a pick 6 to seal the game for the Saints, people forget that the Colts defense hadn't stopped the Saints offense since the start of the 2nd quarter, and even if the Colts had scored a game tying TD on that drive, it was unlikely they were going to stop the Saints offense from going right back down the field for the game winning score.
But having said all that, you still can't deny the fact that Manning has struggled in the playoffs for his career as opposed to how he has played during the regular season, now with a 9-9 record and averaging slightly better than 1 turnover per playoff game. And even though he played great this playoff year, this was the only year, in 10 playoff years in which he played as well as he did during the regular season. I'm not sure that their has ever been a player that is that great in the regular season, yet not so great in the post season, and to be mentioned as the G.O.A.T. your play has to step up in the post season, not step back.