Drush is pointing to the wrong web site
You need to specify your Drupal root to Drush in one of several ways:


Pass --root=/public_html --uri=http://example.com as an option on the commandline.
Set $options['root'] = '/public_html'; and $options['uri'] 'http://example.com' in a drushrc.php (not a good option if you have multiple sites on the system)
Use an alias, and set 'root' => '/public_html', 'uri' => 'http://example.com' (per the article you quoted)
Change your current working directory to '/public_html/sites/default' before running Drush commands (only works if you don't do any of 1 - 3).