You're right - that was a mistake in the quoted essay. Nonetheless, the essay's main points are that Scott Alexander's actions are not consistent with those of someone who values anonymity.
Which is all Scott claims in his new landing page, if I read it right.
Yes, he didn't go to great lengths to cryptographically protect his identity. He's not Edward Snowden, he's just some guy who started writing some essays on a blog, and used a somewhat half-hearted pseudonym probably never imagining it would get popular enough for anyone to look into it further than possibly googling his pseudonym.