Just to clarify, the series is still on hiatus for me as well. I only worked on this chapter to strictly point out there's a different series that people will probably be interested in. I will proooobably do another chapter when Transit's volume comes out.
There's a few possible interpretations involving the story featured here.
I implied this in a reddit comment but it's not entirely clear if this is a literal made up story told to sensei to make a point, or if it's an actual anecdotal story passed down. The essay columns would normally illuminate this but those are volume only and y'know. As such, he didn't change any details in consideration of the other party's account.
Alternatively he deadass doesn't understand the difference between voluntary indentured servitude and slavery in American history. It's possible, I can't really confirm since I don't really have access to a Japanese library or bookstore at the moment. I do know here in America, I have to go to pretty specific libraries just to find detailed literature of the fucking French Revolution. Not having a full grasp of English would affect him here too.
Finally, this could be an edited version of what he heard meant to drive the point home to his Japanese audience. Japanese views on slavery are pretty strange from an American perspective, as you can tell from most Japanese media that dabble in slavery.