But the backing up process doesn't start exclusively as the arm comes above the horizontal and the delivery swing starts. Buttler isn't standing there with the toe of his bat just over the line, stretched out so that when his protractor shows Senanayake's arm is a fraction above the horizontal he can explode down the pitch.
He starts off walking, his bat trails, as Senanayake begins his action the bat comes closer and closer to the line. About the same time as his arm breaches the horizontal, Buttler's forward momentum takes the toe of the bat out of the crease. In this case, he was a touch early and obviously in the wrong; I'm not denying that for a second.
But as the non-striker, your focus isn't on "oh is he going to stop just before he enters the delivery swing to try and run me out", it's on what the striker is doing down the other end. You can't watch the bowler's arm through the action and then track the ball out of the hand; you look towards the striker, watch to see how he plays the ball, then decide whether to run or not.
If you watch the video, you can see that Buttler is looking straight down the pitch, not at Senanayake. He keeps walking forward even after Senanayake pulls out of his action before the delivery swing. Your momentum is going forwards, and you're of the impression that the ball is going to be delivered. You're going to exit your crease whether he bowls the ball or not.