Why is 9-4-2 in NCAA book?
 

7-1-1
A player shall be out of bounds when he or she touches the floor or
any object other than a player on or outside a boundary line. An airborne
player’s status shall be where he or she was last in contact with the floor.

seems to cover the OB status of a player

but 9-4-2
Art. 2. After the throw-in is completed, the thrower-in must touch the
playing floor inbounds before touching the ball.

seems to be redundant. Is there any specific reason for this rule?

It wouldn't be the only redundancy in the rules.

Because the thrower was LEGALLY out of bounds and holding a live ball, but was not violating. This rule is there to clarify what the thrower must do in order to become eligible to touch the ball after passing it. Unfortunately, there were some people out there thinking that the thrower had to establish himself in the inbounds area of the court by touching with both feet before he could catch a return pass. As we know that is not true and the rules writers wished to make this point very clear. So the old one foot in and the other in the air is legal by 9-4-2.