The second postgres redundant - the default database to connect to is the name of the connecting user.
The relates back to your pg_hba.conf concern.
The default pg_hba.conf allows for the postgres user on the system to connect, via the postgres user in the cluster, to the postgres database that is created by default, using a UNIX socket. Among other possibilities "sudo -u postgres" accomplishes the "for the postgres user on the system" portion of the above. The rest of it is handled by defaults within the psql application.
At this point you can connect as a superuser and setup whatever else you require. This is the way Ubuntu means for PostgreSQL to be configured initially - via the postgres O/S user on a local UNIX socket.