I really don't buy this explanation. Federer can make a case for being one of the best servers on tour right now. He has won 79% of his first service points this year, and 89% of his service games. That's good enough for second place on the tour in both categories (behind Raonic and Isner respectively). And it's better than his career averages-- in fact I think those numbers equal his career best. His first serve % this year is also higher than his career average.
It's possible that these great numbers mask the fact that his serve deserts him when he needs it most, but I'm dubious, especially since you'd expect the serve to decline less with age than other aspects of the game.