She is referring to the idea of a Hatstall - where the Sorting Hat can't easily decide between students and their houses.
I had said that examples include Harry, who wasn't an easy choice between Gryffindor and Slytherin and Hermione, Professor McGonagall and Professor Flitwick, who were all considered for both Gryffindor and Ravenclaw.
In actual fact, as the Pottermore writing explains, only McGonagall and Peter Pettigrew were true Hatstalls because they took so long (over five minutes/in Pettigrew's case the stall was obviously between Gryffindor and Slytherin). The others were 'near Hatstalls' (thank you @Janus Bahs Jacquet).
On Pottermore, in some cases, the final question is "Which House do you want to be in?" And you are presented with two choices.
I think Rowling's joke that they are incredibly rare is about the apparent contrast between Hufflepuff and Slytherin, but a couple of friends of mine had this on Pottermore. They came to find quite a strong link between the ambitious, strong-willed values of Slytherin and the truth/loyalty based strong-willed values of Hufflepuff. A Ravenclaw, in contrast, could be considered quite apathetic and remote.