AA was the better "pure ball striker" the way I understand the description. He centered the ball more often and markedly so for how close to the bounce he would take the ball on the rise.
That isn't to say that AA is the better player or even better, overall, off the ground than Fed. You can't view the two players being compared in a vacuum. Fed did and does attempt far more varied shots, in terms of speed and spin, off the ground than AA ever did, from the far greater amount of topspin off either wing to the severe chop backhands he mixed/es in to his shot choices. For what he attempts and succeeds in accomplishing off the ground Fed truly can't be expected to center the sweetspot nearly as often as AA did.