Why can a new user select from any table?
Also addressing the question in the comments.

Role public

Per documentation:


  The key word PUBLIC indicates that the privileges are to be granted
  to all roles, including those that might be created later. PUBLIC
  can be thought of as an implicitly defined group that always includes
  all roles.


Bold emphasis mine. Membership in public cannot be revoked (or granted). You can only revoke privileges from public. In your case, to block out all roles without explicit privileges, like @Robert already provided: 

REVOKE ALL ON DATABASE testdb FROM PUBLIC;


Role postgres

The database role postgres is a superuser by default. You can take that away from postgres - but that would be unwise: everybody (including some client programs) expects postgres to be a superuser. You can also create more superusers (careful with that!). Superusers don't need privileges. The manual once more:


  It should be noted that database superusers can access all objects
  regardless of object privilege settings. This is comparable to the
  rights of root in a Unix system. As with root, it's unwise to operate
  as a superuser except when absolutely necessary.


As to your question:


  Is it true that if \l or \dn+ displays empty access privileges, that
  is the same as postgres=UC/postgres, =UC/postgres


Not exactly.


Privileges are granted by the owner or a superuser (or a role that was granted the privilege to do so). postgres=UC/postgres would mean postgres granted it, which is not accurate for empty privileges, but the effect is the same in a default installation.
However, public gets no privileges for new schemas and only CONNECT and TEMP (can create temporary tables) privileges for databases by default, not the CREATE privilege (cannot create schemas). That would be something like =UT/?? according to the list of possible privileges - the manual once more:




        r -- SELECT ("read")
        w -- UPDATE ("write")
        a -- INSERT ("append")
        d -- DELETE
        D -- TRUNCATE
        x -- REFERENCES
        t -- TRIGGER
        X -- EXECUTE
        U -- USAGE
        C -- CREATE
        c -- CONNECT
        T -- TEMPORARY
  arwdDxt -- ALL PRIVILEGES (for tables, varies for other objects)
        * -- grant option for preceding privilege


You can change default privileges. I wrote more about that in the related answer mentioned in the comments:


Grant privileges for a particular database in PostgreSQL


But those default privileges only apply objects created after defaults were changed. So what does an empty ACL entry mean? The manual on the system catalog pg_default_acl:


  Note that when an ACL entry in another catalog is null, it is taken to
  represent the hard-wired default privileges for its object, not
  whatever might be in pg_default_acl at the moment.


Bold emphasis mine.