If no cache handler is setup or defined, Authcache will fallback to Drupal core
database cache tables and "Authcache Debug" will say "cache_inc: database"
But in fact it seems that you need to add the path to authcache.inc
This is what worked for me:
$conf['cache_backends'] = 'sites/all/modules/contrib/authcache/authcache.inc';
Should I patch docs to indicate this or should the module be fixed so in fact the setting is not needed?