Then how did he win 4 slams? Don't say "because he was good enough to play injured and still win." Nadal was imperious on clay where Djokovic (a pretty good clay player himself) couldn't stop him. He could, however, stop him in 2015, when Nadal was actually recovering from injury. Also, Nadal couldn't have had a dominant 2013 if he were recovering from injury.
Frankly, this seem like "Djokovic's peak doesn't count because Nadal didn't win at that time" to me. We could say the same about pre-2011: "Djokovic was suffering from stamina issues. That means it doesn't count."