I must admit I have never seen any interviews with her or anything, but: How likely is it, that a Fe-aux writes a book series with a main cast that almost entirely consists of Fi-doms and -auxes, with the occasional Te-user? Most people write about what they know well and with what they can identify, don't they? Also, looking at her high-Fe characters: Dumbledore is highly problematic, Grindelwald is even more evil than Voldemort, basically magical Hitler, Peter Pettigrew is cowardice come alive, Molly Weasley favors some random kiddo over her own son, and I don't even need to start about Umbridge, right? Tentatively throwing Petunia Dursley in there, too, I haven't given her that much thought but it seems likely. Lupin is fine, though.
I guess what I'm trying to say is... if she's INFJ, why does she seem to not value Fe at all? Why does she seem to think Fe-users can't be heroes?
Also I see a lot of Ne in her worldbuilding. There are so many shiny and fascinating ideas, especially in the first books, but she doesn't put them to purpose. She doesn't follow through with them. So much unused potential.
Not saying she can't be INFJ at all, but this keeps me wondering.