JK Rowling was trying to make the point that slaves should be slaves and enjoy being slaves, and the ones who want to be free are a rare exception, and anyone who wants to free all slaves is silly and cringe.
But even if you don't take JKR's views as pro-slavery, it's still pretty ironic that the rhetoric of happy slaves is taken from literal slavers from history.
You might also be interested to know that The Lion, the Witch and the Wardrobe was made as a Christian propaganda story. Aslan is Jesus.
I can believe that the origins of this trope are antisemetic, but I don't see very much reason to think that any antisemitic mindset was in JK Rowling's head when she wrote it.