I take this at face value. She said she is an INFJ.
We can't ask her, "Did you take the (notoriously inaccurate) Humanmetrics test, though?" (We could, but it would be rude to ask, and would she reply anyway?)
Both her personal traits and some traits and themes in her writing could place her in the INFJ category. They don't make her a shoe-in, but the idea that she could be an INFJ isn't outlandish.
Her writing is "very Ni". IMHO, that is what makes her work truly good and what convinces me she may be an INFJ more than her behavior. She uses tons of hidden symbolism, and connects clues with patterns. She draws from many different mythologies, Classical, and Classic literature. Her writing may seem stuffed with random filler and descriptions, but all that filling is stringing the clues together. She uses certain words over and over. It isn't because her vocabulary is small. Those repeated words are clues. She scattered information everywhere in the stories, and it connects like a detective’s board. If you connect the dots they give away the entire story very clearly. It is cool.
For example, she had a tendency to hide horcruxes in descriptive paragraphs where she is explaining all different objects in a room. She will go on and on listing objects and hide the horcrux in the list. The clue is repeated.
In book 5 she wrote about weather in a way that connected the story to a myth about Zeus and Typhon that told the reader there were horcruxes (not by name of course, but it gave the idea away) and that a horcrux was in a cave. The same horcrux was listed in a descriptive paragraph in book 5. The plot and horcrux subplot were spelled out in books 2 and 5 for readers who followed her symbolism. There were also clues about horcruxes in the "ship" between Ginny and Harry that related to his scar and the diary. When these clues were discovered, Shippers coined the horcruxes "soulbits". (I wasn't a shipper. It was a little too mushy for me and the ships seems obvious due to the way JKR wrote the relationships, tension, etc.)
It's super easy to tell who will die in the books by the 5th chapter.
In the last book she practically hits readers over the head with clues that Dumbledore is gay. It was obvious to me, but I understood why she didn't blatantly write, "Dumbledore is gay," in the text at the time. It was still controversial, and Potter is a children’s series. (Very Fe of her to avoid taking that risk.)
Similarly, I'm always surprised when more conservative readers get upset with her for her liberal points of view on Twitter, when the entire Potter series is about a resistance fighting evil ala WWII. She makes constant, blatant and sneaky symbolic references to liberal political ideas, WWII, political literature and history. "Equality" is a prominent theme in her work.
I can go on and on giving examples.