You are right they aren´t fighters, but this didn´t held Oda up from giving them 1on1 or 2vs1 battles in Alabasta,Enies lobby, two big arcs.
So they actually show such things, Usopp as example had also 1on1 fights, Luffy vs Usopp was one of the greatest battles between a strawhat member.
And you said it, Nami get Zeus and Usopp get haki grow in DR.
Since that I was hoped to see how they growing up with their new learned ability.
This is a arc of 400chapters of build up, for Nami and Usopp it would be important to let them grow up in this battle, going over their limit like we see in Alabasta or Enies lobby.
And their opponents where get is really worse,Pageone literally not have any move attack, Ulti just use one attack(named one) and the second one was never used...
For all four characters, such a rushed battle is really disrespectful, Nami and Usopp fans wait for so long to get this?
Really disappointed.