For whoever in the future might look for this:
This is a typo-like mistake made by the author I believe. The example code was written with legacy subprotocol and subname while the author’s true intention is to use the preferred dbtype and dbname.
The paragraph right below the example code says:
In the preceding example, we’ve defined a connection for an instance of the PostgreSQL database by specifying the database type suing
:dbtypekey and a name using the:dbnamekey.
So we should really use these two instead.