It’s impossible for them to get to Onigashima. It’s a flying island, they couldn’t even get up a waterfall without help
In Act 4, the Big Mom Pirates are split from the Beast Pirates to fight other opponents. I could see Oda making the match-ups sequence longer in this arc if he doubled the norm for flashbacks for Oden (others were 8 but Oden's was 16).
I made my second comment before you replied. If the Big Mom Pirates don't do anything, I'd be disappointed.