look I'm. not leaning toward sanji having it but oda chose the name of this volume knowing most of em in there are Conquerers, while on the other hand having a names of characters for volumes doesnt make it a must for the character to be there seeing how oda has this plan of making these covers one picture as a whole.. what odd is that this cover has most ever confirmed user (based on the manga speculations)of COC.. with the exception of Sanji who isn't confirmed to be a user of the that skill.. but this volume's name coincides with most of what's being displayed in the Coverpage.. so the likeliness of sanji having this haki increases...
Could Sanji have CoC? Yes ofc, that's for Oda to decide
But using a Volume Cover as Evidence? Ofc not, the best we could say is that it could be a Foreshadowing
Evidence is when it's something that if Story Ends & Sanji doesn't show CoC, we will call it a Plot Hole or Inconsistency
But Sanji having CoC or not, doesn't damage this Volume's Cover Choice at all
Just like there are many Volumes named after Characters who don't appear on the Cover
Not to mention that Oda already named a Volume "Natural Born Kings" & Hancock's Sisters appeared
In my opinion, Oda simply wanted to Draw Luffy alongside Zoro & Sanji while other Two Volumes have Kaido & BM with Half Crew
But the Cover seemed empty, so Oda added Yamato & Ace too
Anyone who says that Sanji was put there because he have CoC, then they must say Usopp wasn't put there because he can't have CoC
Which is wrong, cuz everyone knows Usopp is gonna get CoC, since he lied about it
And if he doesn't, then that's when it will create a Plot-Hole cuz all Usopp lies become True