BIG asterisk to that 15 Djoker win over Nadal, even cali knows Nadal was a shadow of himself that year, didn't win a single big clay court tournament before RG and was beaten regularly on clay that year.
the closest Djoker came to beating an in form Rafa on clay (at RG) was 2013, that was a classic and Djoker had his chances. Tactics matter, Nadal has made slight changes to better fend off Djokovic, mainly his down the line fh. This was shot that bailed Rafa out in that classic 2013 RG semis, he hit some ridiculous down the line fh;s which djokovic couldn't handle or even react too. Without that shot, Djokovic would've won that 2013 match as the nadal fh to djokovic bh wasn't working for Rafa before then. Rafa used that down the line fh against Novak in this year's Madrid final as-well, Novak was tired but that shot has changed the dynamics of Novak vs Nadal on clay, for sure.
Nadal has made slight adjustments to counter djokovic over the years, it's why the wimbledon semis last year was so close; Djokovic, in the past would've beaten Nadal easier on grass. Similarly, Federer has made some adjustments with his tactics vs Nadal, has helped him a bit. These 3 are GOATS for a reason, they can make adjustments, takes years but they can. The only lack of adjustment in this trio is Federer vs djokovic, fed has been owned by Djokovic lately, he hasn't yet been able to make adjustments. Djokovic himself has learned to play Federer, by going to Fed's fh more and not panicking when Federer gets hot, just staying the course, waiting for Fed to get back to normal.