Casey's defense won a championship and they went through the four best individual offensive players in the NBA to win it.
To say he didn't have the tools on the squad isn't accurate either though. Chandler, Marion, Kidd are all elite defenders. Terry, Dirk, proved to be quite reliable and they played very solid man and off-ball defense throughout the playoffs, but these two stepping it up would have more to do with their own maturity and the team policing itself then it would one defensive coach.
And as a defensive coordinator, there hasn't been a coach yet who has had the success Casey has had at implementing a zone on a game to game basis and being succesful at it, that's a notable achievement.