It strikes me as somewhat odd that when someone tries to bring slavery into a global perspective, specifically to the end where they wish to abolish it on a global scale; where it is clearly a larger issue, where people do heinous things to other people based on accidental qualities (read: for arbitrary reasons), that he gets dismissed so quickly.
Slavery is not a white institution. It is an institution of humanity. A horrendous one, at that. I believe that was the point. So no, slavery 200 years ago and slavery 2000 years ago are not different whatsoever. Maybe slaves 200 years ago were shipped from their native lands, tortured, beaten, worked to death, or hung -- and so were slaves 2000 years ago, shipped from their native lands, tortured, beaten, worked to death, hung, fed to lions, or in other cases entire villages massacred, etc. The issue isn't slavery specifically, but what slavery represents: dehumanization.
Slavery, murder, etc, all represent an unabashed dismissal of the dignity of human life. If you are the target of racism, dehumanization, or what I'll call from now on 'hate' for the sake of not repeating a list of qualifiers every time, -- if you're subjected to arbitrary hate, it doesn't matter if you're african, european, american, asian, or a combination thereof; it doesnt matter if you're a christian, a muslim, hewbrew, catholic, zoroastrian, pagan, whatever; the point is that the greatest attrocity that can be commited by man is to dehumanize another man, to strip a person of their dignity and security, and to (whether by degrees or immediately) make their lives hell until ending them.
If anything, I would have thought that Iron Horse was making a good point, that rather than focusing on the 'Hate-issue-du-jour' like France, or the American South, we should be focussing on all of them. All of them. Giving one group a privelige over another is exactly the issue. Saying 'stop hate in France' or 'stop hate in the US', though it may infer that you wish to stop hate globally, does not state it.
Since we're talking about logic, too; if you haven't been given a peice of information, it is not logically sound to infer meaning because that leads to falsehoods. Slavery is analagous to dehumanization and hate. So, to that end, rather than making inflammatory statements that are specifically directed towards the end of 'minimizing crimes' or whatever bullshit that was; it's completely absurd, because that isn't what Iron Horse was doing.
So, to that end, Deep, answer your own question. Is it about 'minimizing the heinous crime of the institution of slavery in the history of the US'? No. It's about 'minimizing the heinous crime of the institution of slavery', period. Minimizing it in the future, by raising awareness about the all-encompassing atrocity it is. Americans aren't the only ones guilty of hate, as you imply by your Holocaust examples. Hate needs to end. That should be the only statement extracted from anything Iron Horse said.
I'm not sure how 'Stop Slavery In All Forms Now' equates to 'racism in certain other forms is okay, and therefore I'm a racist biggot'. Where's that red herring now? We cannot change the past, but we can change the future.
Edit:
Sorry, I feel I haven't been clear.
What Iron Horse said is this:
'Every Instance of Slavery is Unacceptable'
So, I'm at a loss to understand how he could be interpreted as saying 'some instances of slavery are more acceptable than others' when that is clearly not what he said.