Here's the text. I don't see much to argue with:
Acknowledging that the magnitude of victims, geographic reach, duration, commodification, State and corporate participation, and deliberate dehumanization of enslaved Africans and racialized chattel enslavement of Africans produced such enduring global consequences, including the large-scale destruction of African societies, the demographic re-engineering of continents and the entrenchment of racialized inequalities that continue to structure international relations, thereby warranting recognition of the transatlantic slave trade and slavery as the gravest crime against humanity,
1. Diplomatic language is different than ordinary English. It is derived from French, which was the "official" language of diplomacy for centuries. Now there are six official languages of the UN and much of the work is written in English, but it's UN-English.
2. I would say that the claim here is that the slave trade is in the category of worst crimes against humanity. The Rwandan genocide, for instance, was deemed among les crimes les plus graves. That's not a claim that it was worse than anything ever. It was a claim that it's the most serious type of crime.
I know this because I had a student who was a prosecutor for the UN Tribunal for Rwanda (a Rwandan guy) who explained it. He was getting an L.L.M. degree and was required to take intro corporate law. But the language was a barrier for him, and anyway, what did he care about fucking corporate law? He was a war crimes prosecutor. By the numbers, he failed the course but I passed him anyway -- the only time I ever did that, and I think it was justified.
3. I suppose you could interpret it a bit more strongly: when you judge crimes against humanity by the factors listed, then slavery is the worst. And it's hard to argue with that, given the criteria. The Holocaust was four years, maybe a decade if you trace it back before the Final Solution. The Khmer Rouge was 3 or 4 years. The Rwanda genocide was weeks long. Slavery was centuries long.
I agree with you that ranking atrocities is not helpful, but I do not think this resolution is doing that. Some people will use that as an excuse for voting against it, when the real problem is the call for reparations.