> And many outside the USA would disagree with your position that a 200 year old document, written in part by slave owners, is the sole source of truth of our inalienable rights.
Their disagreement also is meaningless, fortunately.
Whether the documents were written in part by Martians or slave owners doesn't by itself alter the words on the documents, as such rationally what is solely to be judged is what was written, not how people feel about some of the contributors. If the world went by the standard you're floating, the last several thousand years of history and all successful political systems of human liberty have to be necessarily obliterated, including New Zealand's entire political system and system of law, which is inherited from a thus poisoned history involving European slavery and conquest.
The US, as with most of Latin America, inherited enormous European slavery and has spent its entire existence dealing with the fact of that history and its intense consequences. What has New Zealand confronted that compares to that? Nothing remotely close (and yes I'm familiar with the history of New Zealand).
> and yet somehow a country with a far better track record in human rights than the USA.
The benefits of being a tiny, largely irrelevant westernized country formed merely a century ago and long after slavery was outlawed in the West. New Zealand's existence has been an exceptionally easy and sheltered one compared to most every other nation, and good for them.
What responsibilities globally does New Zealand have? Practically none. It's free to not matter, in the best way possible. It doesn't have to make hard decisions that risk altering the world. It's like a cute little trinket country, lots of preaching and little responsibility. It never has to step inbetween two warring parties going at it in a civil war and decide which side to support, who is right and who is wrong (North Korea attacking South Korea, North Vietnam attacking South Vietnam), or choose not to get involved at all and have that similarly be judged by history just the same as a superpower capable of intervening (and arguably with a moral responsibility to do so in some cases). It doesn't have to decide if it's rational to launch a war to stop a genocide of Muslims in Europe (Kosovo). It doesn't have to decide whether to protect Ukraine against an invading Russia. It doesn't have to decide if it's worth going to war with China to try to save democratic, peaceful Taiwan. It doesn't have to make a decision about maintaining or not maintaining a global superpower military (which comes with severe, inevitable moral consequences whatever direction you choose to go with that). It never has to make any globally consequential decisions what-so-ever, decisions that can remake the planet; it can be a very nice, easy place to be, the life of a sheltered, small population island that rides on the prosperity and protection of other larger successful nations.
Dropping context around the birth & existence of New Zealand and its particulars, is convenient and makes your premise very unrealistic. The US was born into a context of European sin and had no choice in the matter. Make any other decisions around the founding documents at the time and you don't get a US to begin with (as demonstrated by the civil war that it took to smash slavery in the southern states a century later). Which simultaneously doesn't excuse any mistakes the US has made since then, however context always matters, and New Zealand has had a trivially easy existence by comparison. There isn't a single nation of global importance without some terrible history behind it and there is a reason for that (it's impossible in actuality; only in theory is it not).