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.
The parent's take is a bit too literal, but at least it's her view on human rights activism. That view has demonstrably gotten more extreme post-pandemic.
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.
The Harry Potter series tried and mostly failed to push a bunch of weird political stuff. Slavery is the most noticeable but there was also racism mixed in. The black token character is called Kingsley Shacklebolt. The Chinese token character is called Cho Chang. The war is overtly world war 2, and Voldemort is Hitler - he wants to kill everyone who has impure blood. The Minister for Magic mirrors the British and American leaders who stuck their head in the sand about the war. The banks are run by goblins with big noses.
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.
That's a real stretch. She copied a modern day trope, not some ancient myth with unclear origin. Even if we bend over backwards to believe that she wasn't aware that the trope targeted a specific demographic, there's zero chance that her publisher failed to catch it too. And also the entire movie staff that chose floors with seven pointed stars for the bank. Also also consider that this trope is far from an isolated case in the whole series.
It just doesn't make that much sense to me. "And also the entire movie staff that chose floors with seven pointed stars for the bank" - so the entire movie staff is antisemitic? And what is the connection between a seven pointed star and Judaism / antisemitism anyway?