Mate, let me clarify one thing
If Nadal goes on vacation for 11 months, come back on tour losing 6-0 6-1 on monte-carlo to Raonic, I'd still pick him as one of the favorites in Roland Garros. Guy only lost 2 (morally 3) times there. Period. I think that Djokovic had a slight favouritism over him, but he was "lucky" in the draw by having Thiem to face Djokovic in the semis and not him. I truly believe that Novak would've won that FO final had they met, at least he would have a slight favouritism.
No, he didn't. Betting odds had Nadal as the favorite. Based on what was Djokovic the favorite actually?
Nadal is 6-1 against Djokovic at RG and has won 12 RG, while Djokovic only 1. Nadal also destroyed Djokovic in Rome with a "bagel" (losing a set 0-6, something Djokovic never did to Nadal).
No way Djokovic was favorite over Nadal to win RG. When Nadal is playing good on clay, he depends of himself, exclusively. Djokovic has never defeated a well-playing Nadal at RG, and Nadal was already playing good in Rome, defeating all his opponents with a "bagel" except Tsitsipas.
If Djokovic was unable to defeat Thiem, let alone Nadal who destroyed Thiem in the final. Djokovic was playing 100% focused and in non-windy conditions in the second day of the SF and still couldn't defeat Thiem.
(You can argue that Djokovic was tired in the Rome final, just like you can argue that Nadal lacked competitive rhytm in the AO final). But the fact is, after the 2019 AO final, Djokovic is the ABSOLUTE favorite for the AO 2020. Analogously, Nadal was already the favorite for RG, but after the Rome 2019 final, Nadal was the ABSOLUTE favorite for RG 2019.