Though in terms of peak level, I think he may well have matched the others at the US Open (2010 and 2013), Wimbledon (2008) and Australian Open (2009 and 2012 where he lost the final)
In total he's lost 7 slam finals outside of Roland Garros, so I don't think it's as big a stick to beat him on.
Federer has lost 10, Djokovic has lost 9.
If Djokovic manages to win as many GS as Nadal, then it shouldn't even be a discussion on whom was better considering that Djokovic outside of AU would be quite better than Nadal outside of FO. Let's say (for the sake of argument), Novak wins another 2 AU, 1 Wimbledon and 1 US Open. Then his titles would look something like: 8 Australian Open, 5 Wimbledon, 3 US Open, 1 French Open, compared to Nadal's 11 French Open, 3 US Open, 2 Wimbledon and 1 Australian Open. The distribution of Djokovic's trophies would be better.
Of course, easier said than done, but yea, I think that if Djokovic's matches in GS Nadal, then he should be considered greater. Same is he manages to reach Federer considering that he had a higher peak *. Also, Djokovic leads on head-to-head vs both (although it is extremely narrow, 1 victory more than Nadal, 2 more than Federer). If his head is on the right place, he has every chance to win another few GS.
Would suck though, always had the fear that Nadal - not Djokovic - will catch Federer. To be fair, Nadal might catch Federer too, he'll just win French Open 2019, 2020 and 2021.
* Djokovic's peak was insane, in 2015 until FO of 2016, he won 3 Slams, 6 Masters, but more importantly, he defeated 31 top 10 players. He went 7-0 against Nadal, 6-3 against Federer and 9-3 against Murray. Yes, his % was slightly lower than Federer's 2005, but in all other aspects he was better, including defeating twice as many top 10 players.