You haven't demonstrated it to be nonsense, you've just bemoaned that it used ethics.
Merely the OP demonstrates that, on several relevant points, you are woefully incorrect. It isn't "far worse than anything we can imagine", or whatever virtue-signaling platitude you'd like to use instead. We have data, we have artefacts, we have anecdotes etc.
In fact, the whole thread demonstrates what you are saying is nonsense.
You seem to have missed the key word: slave. Something being less awful, still does not magically make it good.
"Good" is an extremely slippery term that could mean a myriad of things, so you'll have to define it.
That fails in any way to absolve the choice of involvement, a choice which further incentivized slavers.
Contextually, slavery was rampant throughout the world -- you need to acknowledge this. Perhaps, in a similar vain, people of the future will look back on our current treatment of animals, in that we use and kill them for their flesh. Morality is born of pro-social group consensus; it isn't objective.
We should be applauding the Americans of past for evolving morality into a better state, rather than condemning them for what was, at the time, normal.
I have elsewhere on this site countless times. In a debate I would bring out my sources to support that bad is bad, torture is torture, and evil is evil. This isn't a formal debate. Plus you're ranting about lack of knowledge to one of the very rare people with first hand knowledge of the horrors of slavery
If you're going to be lazy and refuse to contribute properly, then do so elsewhere.
At least we agree on something. To me a society falls below the threshold of being civilized if slavery is legal and/or socially acceptable.
Again, you haven't contextualised the environment. Similar to how you'd own a dog, Americans bought and owned African slaves. Very rarely, a slave owner would treat his slaves incredibly harshly. In other parts of the world, slave brutality was far more commonplace.
Furthermore, I have yet to see you rail against the Arabs, Jews and Africans for their brutal treatment of slaves.
And as previously stated: "having your fingernails pulled out doesn't hurt nearly as much as you would imagine before it happens, yet that something is less awful, doesn't make it a positive"
Your analogy is nearly worthless, in that serves to show your inability to contribute properly.
Either post substantial data/sources to add weight to your arguments, or get out.