Dr. Irvin Yalom and Grammar

I recently discovered the life and work of Dr. Irvin D. Yalom, eminent M.D. and fascinating writer. Currently I am reading his The Schopenhauer Cure, a novel, which I I am enjoying and which I venture to recommend.
Dr. Yalom is a bit pedantic, which I love, of course (welcome to the club!), and I suspect he dips into all kinds of dictionaries (as we all should) in order to use the right word to discribe his thinking and his chracters' ideas.
But... I stumbled into this sentence: "How did she used to do it?" which made me prick up my ears.
To use is a regular verb: I use a knife to cut bread with. People used stones but now they use toilet paper. This is fine.
Used to, however, means an action repeated in the past, habitual at a given time: I used to to walk but now I don´t.
Usually = suelo: Suelo ir al cine, I usually go to the movies.
Used to = solía: Solía ir al cine, I used to go to the movies.
However the verb is still "to use" and Dr. Yalom should have written: "How did she use to do it?"
Why did the good Dr. make this clumsy mistake? This is a challenge for you.
Do you know the answer?