The separation of the conviction of the action and the ideas themselves comes off too much like the sin/sinner separation narrative religious people often use to loophole out of respecting others rights. "No, it's not about the act itself, but what the act represents!"
I mean, suppose we're not talking about an 'honorable duel'. What about Crocodile's convictions that he should take over the country? What about Doflamingo's convictions in his right to spread pain and misery to others at his behest because he is a World Noble? What about Arlong's convictions in Fishman superiority giving him the right to cause as much pain as he likes to Nami? You'd have to argue that Usopp also respects all these things. Honestly, conviction is a dime a dozen. It's essentially a common trait among the vast majority of a cast. It's not difficult or admirable to be stubborn about an idea, plenty of people do it. All you need to do to have conviction is never alter your position however wrong you're proven to be, and that's how all these guys operate. But they're never commended for it by the main cast. Usopp never goes "Oh, man, Hody is so awesome that he maintains his belief that humans are trash and how he's gonna go out and kill us all! A real Brave Warrior of the Sea!" This is because it is obviously stupid to commend the wrong kind of conviction. Conviction where the person is obviously dead wrong or is simply malicious or flat out stupid. He doesn't commend any of them like he does the Giants, because whereas he cannot respect Arlong for abusing Nami for his belief system, he does respect Dory and Broggy for fighting pointlessly...which in effect means he is supporting pointless fighting.
So, no, glorifying the will and conviction to do X is tantamount glorifying X itself. Usopp is specifically praising the fact that they are continuing to fight pointlessly because they're stubborn, and brushes off Nami's criticism as "you just don't get it". You might have a point if Usopp went "Yeah, it is pretty dumb, but you have to admit, it takes a lot of conviction to keep going after all this time". This would be acknowledging the nuance of the sitaution, and Usopp just doesn't do that. He approves it wholesale.
The idea never made sense because it is predicated on the notion that the path to one piece as a wide, but straight track. That may make sense from a singular crew's perspective, but not when you look at it globally and try to act like everyone stars off in the 4 blue seas. Supposing that you grew up in the New World, you could very well be 1 island away from Raftel.
Just because he appreciates and glorifies the honor behind the duel that doesn't mean he should then appreciate the convictions of every other character, also it takes time for Usopp to fully accept the fight because he even questions why when they start fighting. Then as he learns why he accepts the honor and conviction but still doesn't like violence overall.