That was just Sanji goading on Wadatsumi.
Fighters in the OP world have been shown to be stronger than stone all the way back in the Arlong Saga. Arlong could crush a stone pillar, something heavier, denser, and "more durable" than his hand, just by gripping it. Luffy could destroy stone pillars easily with one punch, and still needed to clobber Arlong to knock him down. What that means is, the body of Arlong was far stronger than a stone column, needing more energy exerted to generate the force to break him. And Wadatsumi is FAR, FAR stronger than Arlong was then, taking a direct Gear 2 punch from Luffy and only suffering a broken tooth(by comparison, one Gear 2 punch destroyed a Pacifista, which pre-skip Zoro that was cutting stone since Alabasta couldn't even scratch). So this argument that stone>Wadatsumi's flesh doesn't apply to the universe of the manga.
Except Pica. Pica isn't made of stone, he's just inside of it. If Sanji's fire spreads throughout inside of the golem, as it would by the nature of fire-based kicks, even if the stone largely is unaffected, Pica is still flesh and blood. Superheated stones=molten earth covering Pica's body, which isn't good for even the best fighter.