Sanji is NOT BOUND by promise of getting stronger through a certain way... his dream is finding all blue... It's never about getting stronger through the HARD WORK.... Zoro's dream is strength related... Luffy's dream is ALSO done only through strength to topple the Yonko... those two ARE NOT ALLOWED to have short cuts.... because their dream is related to how hard they work for it. Sanji's strength upgrade is not related to his dream, so basically. ONLY his dream related stuff should be about HARD WORK... the cooking aspect.. and at no point Sanji took a shortcut regarding food or cooking... and because his strength is not about his passion or his first interest... he can ACCEPT shortcuts...
Just think of it that way... You are playing a game, very challenging game... if you are SO PASSIONATE about it.. you will never cheat your way up, you will try to play in the hardest levels to feel the excitement... that's what Luffy and Zoro feel regarding adventure and fights..
But Sanji isn't even interested in the fighting aspect... for him, he would prefer to set sail, have fun with Nami/Robin.. cook.. find all blue, without a SINGLE FIGHT... so if a fight came, he is not a man who gets entertained from it like Zoro... so if he found a way to win EASIER, faster, more quick, he SHOULD TAKE IT, cause it's not his priority