Okay, wait a minute. I am confused here, and I am sorry for getting a little off topic. If the batter throws the bat ON a ball in play (i.e. already been hit) this is double contact and the batter is out, correct? My DD has had this called on her when putting a bunt down.
You are just misreading the scenario.
They are not talking about a double contact(moving bat hits ball 2nd time) which is a dead ball and an out.
They are talking about a ball being legally in play and the bat being physically thrown (i.e. the bat is carelessly thrown dangerously after the batter hits the ball in play which is going to end up with a warning or bench restriction after the play is over)