Maybe Oda still intends to show Fisher Tiger releasing the slaves in Mary Geoise, but that would be the third time telling the same event.
Anyway, the two reasons why I believe Oda didn't properly show that scene was (1) it would show too much of Mary Geoise too early, (2) it would take a lot of pages to tell an event which is mostly action, the kind of stuff that Oda does offscreen anyway.
Not showing Marijoa could be part of it as well.
I think the ultimate payoff could be in the final war. Imagine the Straw Hats are in Marijoa. Nami and Jimbei come across an enslaved Arlong. It would be full circle for all three of their characters. Arlong was in Impel Down 14 years ago during the Tiger flashback, but wasn't there when Luffy raided the prison. Perhaps he was taken as a slave to Marijoa. I could imagine Nami liberating Arlong from slavery in an ironic twist, demonstrating herself again to be the far better person.
–- Update From New Post Merge ---
It's one thing to say that Hancock may not join even if she's about to return to the story and has a flashback to be told. We've seen non-SH characters with flashbacks, like Wiper, Kyros and Law, after all.
It's another to believe that Oda won't show Fisher Tiger's greatest achievement, and from the emotional perspective of the slaves he was freeing, no less.
As for me, I will double the bet by saying that Hancock's flashback will also cover Fisher Tiger's and Koala's time as a slave, as their three stories intertwine in captivity.
I feel like, if we don't see it told through Hancock's perspective before the war, it is something we could still see all the way up to the point where the Straw Hats actually arrive in Marijoa.