Well winning the last 3 probably has something to do with it, along with 37 year old Federer, 32 year old Nadal but in basically a 42 year old body, and Thiem, Zverev, and those other mugs still being slamless.
I dont even know if I would say strong favorite or more slightly or shaky favorite over the next 2 Nadal and Thiem,but definitely the current favorite regardless of degrees. If it wasnt for his history of frustratation at RG I probably would say strong favorite.
As you said though Rome will be much more telling to the exact odds going into RG for a lot of players though.
I'd say Zverev, simply because these NextGens tend to have 1 good tournament from each season and Zverev is yet to win a big tourney on clay this year. However, Thiem, Nadal and Tsitsi all have legit to some chances to beat Djokovic now.