It's not a conspiracy or anything like you're describing, it's that moderate politics is often just... wrong. Believe it or not, there's sometimes a right answer, and a wrong answer. And if you go "in the middle" you end up being wrong.
I mean, take a small gander a history. There's usually a right answer... and a wrong one. Typically, in the moment, the people don't know that. So some construct middle-ground that they believe seems reasonable. Then, we look back on it, and say "what the fuck were you thinking". And that's how we end up with bullshit like the 3/5ths compromise. Seriously, what the fuck was that?
Point being, simply being moderate or depolarized doesn't save you. Consider that, in some countries right now, moderate positions are pro-genocide. Because that's how far gone the politics in said country are.
Also, as you said: https://en.m.wikipedia.org/wiki/Argument_to_moderation there's a name for it!
I disagree, they absolutely thought it was morally correct because morals progress throughout history.
What I think people miss is that slavers were evil people doing evil things. No, they were normal people doing normal things. And that's much more scary.
There's a particularly annoying form of cowardice that involves refusing to take a position and then pretending this is some kind of moral high ground.
Believing that your position or your side's position is always correct and not bothering with other perspectives is the first step to becoming ignorant.
The second step is to believe that everybody who isn't explicitly of the same opinion as you must be some kind of shill or agent or infiltrator is the second step.
Just to pick one easy example, the confederate states of america employed agents to advocate for peace treaties and cease fires during the civil war. By all reasonable classifications, they were "enemy agents trying to weaken our resolve".
Would some people have benefitted from a peace treaty? Of course. Would everyone have benefitted equally? Obviously not.