Here's my current theory as to how the Straw Hats survive:
After the explosion, it's revealed that the Straw Hats aren't wiped out just yet, but that they were somehow saved by Brook. Brook reveals that he recognizes Kuma and knows how his powers work. They've met in the past, actually. Kuma is surprised, as he doesn't recognize Brook at first.
Turns out that Kuma was involved in the death of Brook's crew, and was actually the one to kill them all. Kuma did this while he was still a young child at the time. We get some flashback chapters showing all this, and see Brook's crew and Kuma, and just how bloodthirsy and horrible Kuma used to be.
Kuma, as it turns out, deeply regrets his past actions as a pirate and is trying to repent for them by hunting down brutal pirates as a Shichibukai, protecting innocents from people like he used to be. But there's something in particular about his massacre of Brook's crew that he regrets, even though they were pirates too and not civilians like many of Kuma's other victims.
So Kuma decides to spare Brook this time. Brook proudly proclaims that he is a member of the Straw Hats now, and asks if if Kuma will leave him the only survivor a 2nd time. Kuma prepares a new bomb, even bigger than the previous one. Then he tells the Straw Hats that he can't just let them go, but with the damage it has already sustained, Thriller Bark might just be sunk when the new bomb goes off. If that happens, Kuma will have to make sure Moria doesn't drown, and between that and all the chaos the sinking of Thriller Bark would cause, it just might not be possible for him to pursue and catch the Straw Hats and Rolling Pirates, if they are quick about leaving.
He warns them that he doesn't expect them to get a lucky chance like that if they ever meet again, though.