I've read this before, an while it does makes sense, the author seems to say that Federer sees the ball this way all the time while others don't, and selects photos to support it. However, a quick google image search brings up many photos where he's looking at the ball in front at contact, and you can find photos of other pros that seem to be looking through the back of the racquet at contact. You can find examples both ways.
I got curious and looked through my sons tennis photos and found some pics as young as 8 where he is hitting out front and seeing the ball through the back of the racquet at contact, and others where he is looking in front at contact.
My theory as that it is dependent on many factors - time at the ball to hit out front, position of the body, speed/trajectory of the incoming ball, etc. I don't think Federer is doing anything magical that other pros are not, but his movement, court sense, anticipation, etc allow him to have more time at the ball and do it more often.
In fact, I found more photos of my son looking through the back at younger ages and less as he gets older. I think this is because the incoming pace is greater as the level rises and he has less time at the ball. At the pro level, Fed has more time than many because of his movement and anticipation.
Well, that's my theory anyway...