That's a huge Chekhov's Gun in all honesty. There is no reason that Oda would have placed that into the story if he wasn't going to make good on it. People could try to argue that it was thrown in to explain how Sanji handled the knives to well, but there was no need for that, as everyone just assumed it was because of his skills as a chef. The fact that Oda blatantly made it known that Sanji knows how to use swords will play a factor down the line.
In fact, there is a question as to how Sanji freed Momonosuke. Those chains didn't looked like they were "busted" apart, but rather cut apart. I said in the spoiler thread that I wouldn't be surprised if the Raid Suit is equipped with an invisible blade of some sort. It would be a pretty epic way of introducing Sanji using a Sword is if we see King fighting against an invisible Sanji, and saw King's sword clashing with thin air, and when Sanji makes himself visible, he is shown wielding a Sword of his own.