[ta_8UgB54G]

Occam's razor suggests that your sample size is too small to make any generalizations about what a typical white American would think about anything. Occam's razor also suggests that the nationality of posters depends a great deal on where in the world it happens to be working hours.

Slavery doesn't come up that often on HN. Responses in those threads are biased towards users who care enough about the subject to want to engage in conversation about them. And I would be willing to bet the comments you're describing don't account for the bulk of thread contents. So you're making an assertion about white people and Americans based on a sample size of, what? Maybe a dozen posters, if that? Bear in mind that posting on HN at all is atypical.

I've got no issues with your plea for empathy but you're not making a strong argument that the comments you're talking about (whatever they are) necessarily defend slavery, or that the people making them necessarily believe that slavery is acceptable, or that they're evidence of what what white Americans believe about slavery as an institution, in general. You're literally arguing from the premise that the racism of white Americans can be taken as a constant.