Sanjis flames are unexplained yet, Sanji has been associated with DFs, spiral power and space by Oda, Sanji is confirmed connected to Vegapunk and lineage factor manipulation. For all we know, DFs may have been made from real living beings, Luanrians for fire theme DFs especially Ace's for example.
Funny how Oda waited until now for Sanji to get Ifrit Jambe specifically too after well over a decade of just Diable Jambe? We had Hells Memories as the only real upgrade for DJ and Sanji generally post TS but now thats all changed ofc....
Zoro and Luffy also have unexplained abilities of producing fire somehow and if you want to treat it as canon, Pirate Warriors 4 lets Kinemon create fire too lol. Kaido was also given ice powers in that too though, so go figure.
But honestly, if Diable or even Ifrit Jambe turns out to be connected to Nika/the Nika fruit, then that is gonna be HYPE ofc!