Also to answer your second question, first off there's no user table in the example so it's a bit confusing. But say we're just comparing a traditional table-per-record-type approach and why this approach still retains the coupling problem.

In that scenario you could join the user table. You'd be overfetching in many scenarios but that's not a huge concern for most people, this is what active record does. However, say you want to get more than just the columns on the user table, then you run into the same issue. Suddenly the query caller needs to inform the query method to include results about some unrelated table. Because datomic is an in memory graph structure, the caller can handle grabbing that extra information without modifying the method or its call signature, obviating the need for this coupling.