The SHs were in no way equipped to take on the Big Mom Pirates in WCI. They got destroyed and saved by Plot countless of times.
Again you cannot remotely compare the stakes of Sanji and Robin's situation even if the scenarios were similar. In Robin's case, they were up against a WG team which was around the same lvl as the SHs, with the SHs being the only people Robin is protecting. Not to forget the full crew was there.
Here Sanji is up against a Super Powered full Yonko crew, with half the crew, with the Baratie, Kamabaka and SHs all in danger. They have absoloutely no way of winning. And escaping would mean the deaths of people of Baratie and Kamabaka, and also immediate deaths of the SHs.
Of course with Oda's big pen of PLOT, he made all the threat dissappear as soon as Sanji changed his mind. But lets not forget that they werent there. Sanji's situation was far worse than Robin, since it was endangering people who had no idea they would be attacked or could defend themselves. Whereas Luffy and Co. were up against similar lvl people.
Robin herself says so.
Robin: "Dont forget, this isnt like before"