It is real, but it has limitations. It depends on the team around the QB as well. A QB can take a team on his back and do unbelievable things, but he needs receivers to make the tough catch, the D to get a big stop, etc. He can only do so much himself.
As an example, Mahomes was great enough to win the Super Bowl 2 years ago with an underwhelming receiving corps, but he did have a strong OL, enough of a running game, and his own ability to create plays to overcome it. However, in this past SB, his OL laid a huge egg, his WRs had some huge drops, and the Chiefs were never really in it. Does it mean Mahomes lost it? No, a man can only do so much.
Plus, I think guys have shown they can develop having "it". Elway is an example. He's on 3 SB losers, but shows the grit and leadership in winning 2 before he retires. Of course, having Terrell Davis helped quite a bit, but that again goes to having the team around you to get the job done.
And there is just lousy luck in some circumstances, Kelly brings the Jills down to kick the game winner against the Giants, but Norwood misses it a foot to the right. History is totally different if Norwood hits that kick. (As a Dolphin fan, one of my fave SBs ever lol).
PS, with all due respect, how does Warner get on your list with Montana and Brady? He won it all once with one of the best offenses in the history of the NFL. Did he make the tackle on the 2 yard line to keep Tennessee from coming back? The next year, he loses it as a huge favorite to an unknown Tom Brady. And then Roethlisberger had "it" more than Warner did in his SB with the Cards.