Ironically, most people using that line are trying to say American slavery was worse than other forms of slavery, as a weird way of trying to make America out to be the worst place in the world.

As you say, slavery is pretty bad and trying to figure out which one is "better" is rarely a productive exercise.

Apologies, I meant my argument against US slavery being the worst. As you say, the common framing is usually a willingness to believe any framing that can support the US being the villains. I truly don't get it.