It's most of his name. Long before his full name became common knowledge, you could already Google "Scott Alexander psychiatrist" and find him almost instantly.

It's his first and middle name. At least that's what he said in the post about shutting down the blog.

It is his real name, and he also used his real surname in early blog posts.

His pre-SSC blogging was, and he’d link those posts directly from time to time.