Well you see, Sanji could use Geppo, so you couldn't call that his air attacks should be weaker. But in result he was send flying by much greater power.
. Same happened with P1, Sanji litteraly was overpowered, by stronger enemy, litteraly while he used his strongest weapons- his legs. How then Sanji's leg didn't broke this time. Well you could see that Vergo hit him into shin which is not Sanji strongest part. Kick itself is stronger than hit from knee hit . Also I'm not Law fan, so I never took seriosly his words. Sanji also think that Nami wouldn't be hurt if Usopp was with her...and Nami would be dead if not Tama for sure. So I think that believing in words that character said never mean 100% truth, or author opinion.