took something off the throw when he saw (as he was releasing his throw) that the runner was not close.

The throw was to the catcher's backhand side & he didn't know that the runner wasn't sliding home, so he instinctively made that swiping motion with his mitt / left arm. Sometimes players lose the ball when they do that, although I think that Smith actually lost it before he swiped at the runner. (I may be wrong about that.) All of which is to say that had the runner slid into Smith's mitt at the plate, who knows whether Smith would have held on to the ball.