There is no such variable like page_cache_lifetime

While debugging Redis integration I have found an odd variable page_cache_lifetime in the D6 core, which is not used anywhere else, just in this single line only in the function page_set_cache(), so it is probably a typo which may introduce serious confusion in the caching logic behavior, because it will force CACHE_TEMPORARY (-1), no matter what value is set in the proper cache_lifetime variable.

function page_set_cache() {
  global $base_root;

  if (drupal_page_is_cacheable()) {
    // This will fail in some cases, see page_get_cache() for the explanation.
    if ($data = ob_get_contents()) {
      ob_end_clean();
      $cache_lifetime = variable_get('page_cache_lifetime', 0);

      if (variable_get('page_compression', TRUE) && extension_loaded('zlib')) {
        $data = gzencode($data, 9, FORCE_GZIP);
      }

      $cache = (object) array(
        'cid' => $base_root . request_uri(),
        'data' => $data,
        'expire' => $cache_lifetime > 0 ? $cache_lifetime : CACHE_TEMPORARY,
        'created' => $_SERVER['REQUEST_TIME'],
        'headers' => array(),
      );

      // Restore preferred header names based on the lower-case names returned
      // by drupal_get_header().
      $header_names = _drupal_set_preferred_header_name();
      foreach (drupal_get_header() as $name_lower => $value) {
        $cache->headers[$header_names[$name_lower]] = $value;
      }
      cache_set($cache->cid, $cache->data, 'cache_page', $cache->expire, serialize($cache->headers));
      drupal_page_cache_header($cache);
    }
  }
}