I think the misunderstandings are because they are targeting a specific audience. They aren't trying to teach you Clojure, Datomic, and Reagent because they assume you are already knowledgeable about it.

Clojure and Clojurescript are the same language (more or less). They just target the JVM or Javascript.

The author is mixing the frontend (Clojurescript and Reagent) code with the backend (Clojure and Datomic) code in the same expression. Then through their magical system and the beauty of lisp, they pull the frontend and backend parts out to serve them separately.