Anways with regards to Sanji's weakness that Ivankov spoke about in this chapter, I think it's almost certainly going to be Sanji's self-sacrificial nature. In otherwords, the fact that he holds the life of those he holds dear to him over his own. This is something that has been recurrent with Sanji since his very introduction into the series.
Sanji was willing to sacrifice his desire to find the All Blue in order to repay his debt to Zeff
Sanji was willing to sacrifice his life for the crew on WCI.
Sanji was willing to take a chance and sacrifice his life for a family that have detested his very exisence since birth.
Also just looking back on Sanji's time with Iva, even though it was a gag scene he and absolutely did not want to acknowledge the picture of his bounty poster as himself he did so in order to prove to Ivankov that he was Luffy's nakama to the point where he was legit coughing off blood. Reckon there was prolly other situations like that on Kamabakka.
Also if this is true, as much as I hate to give Oda credit for shit coz he's so such a chaotic and lazy writer, Longinus being pit against Sanji is an intersting parallel, as it is said that it was the spear of Longinus that pierced Jesus Christ to confirm his death, who also happens to have a self-sacrificial theme around him. Still though, name drop alone is not enough. Oda's gonna have to deliver on the Longinus plot even if only from a narrative-standpoint.
I do wonder what direction Oda's going to go with Sanji though. One of the tings that makes Sanji, is the fact that he's kind to a fault so will be interesting to see how Oda resolves this conflict.