Note que apenas alguns segundos depois que as portas se abrem, elas chegam em massa.
The door opened to reveal a blood-spattered, but not noticeably wounded, River Tam, holding a Reaver blade in each hand. She was standing in a very big sea of Reaver corpses.
Several curses went through Mal's head, all of them woefully inadequate.
Then Mal noted the grappling hooks in the wall, and was thinking this whole thing might not be as over as he thought a second past.
Sure enough, the walls were ripped away with a crack that near deafened Mal, to reveal a whole passel of Alliance soldiers.
River may've done a job with them Reavers, but she also cleared the path for these more coherent folk.
Por que eles precisavam atravessar a parede, a resposta mais simples é que Mal e sua turma já tinham desmoronado a passagem principal com suas granadas. A parede divisória era evidentemente a maneira mais rápida de passar por eles.