Mats Wilander believes that Djokovic's 10 Australian Open can easily be compared to Nadal's 14 Roland Garros championships because of the surface difference.
Wilander's point stands on the fact that winning 10 Australian Opens is tougher than winning 14 French opens due to the surface. Most of the best players in the world know how to play well on hard courts as it's the most common court variant and the majority of tennis is played on it, including two majors.
Clay is a widespread surface as well but not all players know how to play well on it. So in essence, Wilander feels that Djokovic faced a tougher challenge in his runs than Nadal. Players simply offered more resistance on hard courts than they did on clay courts. It's actually true because the Serbian had to play plenty of five-setters in finals to win them.
Nadal had an easier job at Roland Garros as even the best like Federer or Djokovic rarely proved a match at Roland Garros. But it also speaks to how much better his game on clay is than everyone else. That's why he's the king of clay.