I’m a little confused about the point being made.

Are we implying that McDonnell is a neo-Confederate and by extension pro-slavery/closet-racist?

That’s what it looks like, by making the connection first between McDonnell and the Neo-Confederate groups through this oath, then the comments pointing out that one of the most apparent characteristics of the Union vs. Confederates was their respective stances on slavery.

I don’t really have an opinion on McDonnell, I’m just not sure if I am making the right inferences here.