Sanji doesn't need to be saved though. Nothing is forcing him to go other than taking care of his own personal trouble
He said he'll come back
True..
But it's not that he NEEDS TO be rescued per say, but it's a safe bet to say that he's gonna need back up and some support from friends, during a time of rejecting a Yonkou's request to marry her daughter. And his Family of Assassin's request to marry that Yonkou's daughter.
I mean come on.. the situation speaks for itself. Sanji is not going to be able to just simply walk away freely from this without a fight. And if he's all alone the odds are surely against him.
This is where strong Nakama come in.. real friends.. they're not going to let Sanji go through this alone. It's not a "rescue". It's support / back up.
And Zoro of ALL people (after his speech in W7, with the whole Usopp ordeal.) Is not going to deny or say no to his captains orders on departing to a certain island and going to help their very own chef / friend / 5th member to join.. Zoro and Sanji have they're fights and rivalries, and disagreements and arguments or what not, but Neither of them are going to be disloyal to Luffy. It's just not gonna happen.