My hackernews account here is "anonymous", but not carefully so. I'm sure any dedicated reporter could figure out who I am (I have no pretensions that I would ever merit attention like this). Still, if one decided to out me after I asked them not to AND it didn't even really advance the story, I would indeed feel violated. And I'm not a therapist with patients to consider, I'm an IC FAANG grunt.
I never made the appointment because it would have been really difficult to separate the writing from the person. However, that is on him! Plenty of mental health professionals blog and see patients.
In the past if you searched for "Scott Alexander" you could find the real person but if you (or a patient) searched for his real name they wouldn't have found the blog posts.