Your "defense wins games" statement would have merit with a opponent that wasn't widely acknowledged as having one of the top 2-3 defenses in the league. Besides, just using opposing teams FG% as the only statistical criteria for saying they are the best defense is not very realistic. While they were allowing a worse percentage to opponents, they were also giving up more points because they turn the ball over almost 20% more than the Pistons. They also tend to be a little more reckless on D, allowing opposing teams to get to the line 25% more often than Detroit.
Again, good team, might even sneak a game or two away from Detroit with some breaks.
