This is a common piece of misinformation. While the parties have changed over time, republicans have transitionally always been representative of natural rights and on the right, while democrats were more focused on egalitarianism and other more left-wing topics. Freeing slaves was a republican policy because it was focused on natural rights, the civil rights act takes natural rights while producing a more egalitarian society (i.e. to ensure everyone is allowed in your water fountain I must require it through law, and close your store if you refuse). Certainly the parties have shifted over 200 years, but I don't think you're fully caught up on your history.

That said, this article is about removing references to slavery in national parks, so I'm not sure where your point is going.