Typo in replacement variables in admin page

You have a typo in the first form element on the admin page:

   $form['htmlpurifier_introduction'] = array(
    '#markup' => t('<p>This page contains global settings for all HTML Purifier enabled filters.  If you are looking for specific filter configuration options, check <a href="@format">the format configurations page</a> and select the specific format you would like to configure.</p>', array('@filter' => url('admin/config/content/formats'))),
  );

You use the @format replacement variable in your translatable string, but you use @filter in the array of replacements, hence the replacement is not executed...