The flashback was to explain Brooks half of the events. Why he knew about the HKs, why he believed Gunko killed her father, how he went from battle convoy leader to pirate.
Manmayer wanting Shuri back isn’t outside of the established norm, almost 1:1 with Garling wanting shanks and shamrock. Remember, in the HKs eyes Shuri was a god among garbage. It’s very possible that Reuven lied about the war with the WG. He said they wanted 1000 slaves, it’s possible that the only thing the WG demanded in place of tribute was Shuri and Reuven refused.
Gunko will fill in what happened after they left when Brook and her talk. (Probably a one page flashback)