One of Federer's great strength is getting the return in play so I'm not surprised by the stat. People are comparing Agassi to Federer in this way and pointing out that Agassi is inferior to Federer in this way, which is true. However the key stat to me is how often a player can break serve and while Agassi could be aced more than Federer he did break serve more often by percentage than Federer. That being written it is a very impression feat by Federer.
I would tend to think a Ken Rosewall in the past was very hard to ace also. Rosewall could get a lot of seemingly unreturnable serves into play.