I thought of an even simpler example for JR.
And with him keeping it simple is important!
During normal play the ball is live and in-bounds, but gets batted away and bounces toward the stands. The ball crosses the OOB plane of the sideline, but A1 jumps from in-bounds and saves it. Did the ball ever go OOB? No, if it did it would have been a violation the whistle would have blown. Did the ball cross the plane? Yes.
So just because the ball is on one side of the OOB plane or the other doesn't mean that the ball is inbounds or out-of-bounds. It simply means that the ball is on a certain side of the plane. So JR should have written that technically the ball had to cross to the in-bounds side of the throw-in plane or else it wouldn't have been legal for the defender to grab it.
The ball's inbounds/OOB status is determined by where it last touched or is touching a player or the court. What side of the plane it is on has nothing to do with it.