Most in game decisions, in and of themselves, don't change the win expectancy of the game very much. That's a fact.
That said, I do agree that Cora makes some good in game decisions based on the knowledge of his players. I think that he knows how to put his players in the best position to succeed.
Those two statements might seem contradictory, but they really aren't. It's more about managing the players than it is about managing the game.
As far as run differential goes, would it surprise you to know that the Astros are 7-8 in one run games while the Dodgers are 11-14? Both below .500. Meanwhile, those teams are 21-5 and 16-5, respectively, in blowout games. You can tell more about a team's true talent level by looking at their records in blowout games than you can by their records in one run games. Like it or not, the outcome of 1 run games is largely random.
I'm not saying the Red Sox aren't good. They are very good. But they've also had a bit of good fortune on their side. There's nothing wrong with that.