It's not a principal of cook, case there are many cooks who use their hands in fight
So that is exactly just a rule that Sanji made for himself, based on Zeff ideas. Same was with " I will never heart a woman evn if I die/ or my nakama die"...yes Zeff give a base to that, but I doubt that Zeff wouldn't beat crap out of Kalifa, to the sake of saving his nakama Robin. That time, Sanji also said that he will not run away, case that also goes against his rules of "perfect man"...and that was stupid as hell and lead him to Kamabakka kingdom.