Are you sure that's all there was? I've never heard anyone argue that the slaves themselves benefited from slavery. I want to be clear I'm not making this argument myself, only describing a thing I have heard... I have heard someone argue that modern blacks have received a net benefit compared to if their ancestors had been left alone. And the context is about how to calculate reparations. What you described, slaves themselves benefiting directly, doesn't sound familiar at all.
I remembered slightly wrong, it was more complicated than that. A rather right leaning commenter said that if one were to accept that reparations for historical slavery requested by BLM and SJW were valid, then because African-Americans are claimed to be still better off than any other black populations in other countries, the slavers should be celebrated not torn down. The commenter says because this is “morally repugnant” then BLM and SJW are thus wrong. So it’s a case of false equivalence and setting up a straw man Or something (they love their rhetorical flourishes at SSC). So the slavery issue wasn’t actually claimed, but it was set up as a logical conclusion or the opponent’s perceived position, and this was never countered.
