..... Either to try work things out with that Coup de Burst (which will take under a minute to launch, that is, if they can finally found the cola to refill and stuff after a few minutes spent to figure that one out), or make a diversion and escape thru what they can really do at hands?
Besides, Sanji has gotten permission from Luffy to make a counter attack. He probably assume this sort of thing may happen in worst case scenario. If they have to escape from a Yonko subordinates, they need to do some distraction, and fast.
Coup de Burst has just been used earlier in order to escape from Doflamingo. Chopper whom had just used it didn't step in and claim he could, because he too didn't know how to work at it again after having it launch before, like how many barrel of cola is needed or where in the world the cola is stored, etc. (I don't think they need to tell us twice for that, so Brook, or someone had made it clear about it), therefore, Chopper gave a second option; Gaon Cannon, either to attack Big Mom ship for some distraction or in panic.
Though I still have my doubt on who had actually said that he didn't know how to use Coup de Burst. The statement was more like in a calm manner, I thought Sanji might be saying that.