It's about acknowledgement
I made a thread once talking about this, if im not mistaken.
On the same chapter of the colospread in question, Sanji asks Zoro to kill him if he goes out of control. Perhaps thats Sanji's acknowledgement that Zoro can stop him/Be superior to him. Even if, in normal circumstances, they get very close in strength.
Taking a color spread's meaning over the manga that still hasn't reached its end is dumb if you ask me