To start off, it’s expected for power houses like Luffy or Zoro to stand up to an overwhelming enemy while on the verge of death, but it means a lot more coming from someone like Nami or Usopp that would turn tail at the slightest opportunity.
In the last chapter, Nami was offered a chance to be spared. None of her crewmates would hold it against her for trying to save her hide with a white lie, but her faith and devotion to Luffy are greater than her own life.
I can’t help but think this moment was in reference to this little moment the human duo had back in Act 2.
It reinforces that the crew are equally a group of friends travelling as they are a Pirate crew. They are capable of making remarks and jabs at each other, but when push comes to shove, they will stand by their friends 1000%.
But let’s not overlook Usopp, he stood his ground against two Flying 6 members by himself in the effort to protect Nami. He even took two headbutts for her sake, too.
And after he’s put down, his immediate concern wasn’t himself, but Nami, assuring her it’s ok to lie in order to save herself, and later shouting for her to escape.