Zeff is teaching Sanji what the chapter is about in real-time. The doctrine is the technique-as-grit axiom — Straw Hat combat-philosophy in its East-Blue articulation. Every later strongest-man-with-best-equipment claim (Krieg, Bellamy, Caesar’s gas-formulations, Doflamingo’s strings) is structurally inverted by technique-strongest-without-equipment counter-claims (Whitebeard’s Gura-Gura, Mihawk’s blade-discipline, Rayleigh’s Haki, Garp’s fists).