It is cited, but buried half-way into the article. There is a link in it that leads to https://erikbern.com/2015/10/01/nearest-neighbors-and-vector...

The article isn't meant to present a new library competing with Annoy or other state of the art approaches, but instead, describe how vector search works using short code snippets.

I've now updated the article to clearly mention this and also cite annoy.