Out of 14 million timelimes, in only one of them the rat pushes the button to bring Antman back from the quantum realm. Without this happening, the entire chain of events would have never taken place. The rat was the catalyst for the entire movie.
Doc kept signalling to Ironman throughout the movie that there is 'only 1' timeline. He couldn't tell how things would unfold directly, otherwise it would change everything.
Remember when Doc said something like "all roles are important, from the elephant to the field mouse" when Colossus was introduced to the board? Well… This holds far new meaning now. Also funny how he said it was a coincidence that Antman was back from the quantum realm just before the movie.
Who is the true hero in End Game? The true hero is the rat, err field mouse! Coincidence? Mind blown!?