Here’s a good video illustrating Rules 9-6-3 and 9-6-4.
Rule 9-6-3 says that “a player shall not contact a ball which is completely on the opponent’s side of the net unless the contact is a legal block.”
Well, what makes that an illegal block, you ask? Well, that’s what Rule 9-6-4 is all about: “Blocking a ball which is entirely on the opponent’s side of the net is permitted when the opposing team has had an opportunity to COMPLETE ITS ATTACK.” The key part is capitalized. The setter was trying to save the ball from going into the plane of the net and was not attacking the ball, therefore the blocker cannot reach across the net to block the ball.