Using your reasoning, one can conclude that Rowling wanted us to believe that non-magic people are always backward. Which I don't think Rowling intended
Although I disagree with this person, I think the comparison between Susan and Muggles is a very good one.
My view is that Rowling doesn't really hate non-magic people (I can't imagine how she would hate herself and her husband and kids). But I think 'muggles' are to some extent a metaphor for unimaginative people, and I think my commentator is right that so is Susan.
Both Susan and the Muggles are metaphors for non-intuitive, pragmatic, conventional people, and Rowling and Lewis are both intuitive people who are expressing resentment at the down to earth majority. To some extent I can sympathise. I think Rowling's view is richer, because in her story the best wizards (ie the best intuitive people) realise that muggles are people too, and have as much right to live and be happy as anyone else.
There's another element of Rowling's work which is that she expresses the propensity for evil that intelligent and imaginative people have. Her wizard world is dark and full of racism and fascism. I think clever and imaginative people can do more harm than unimaginative people, in all sorts of nasty ways. I think that danger is shown in the Harry Potter books, but overlooked by some of Rowling's starry-eyed fans.