Refreshless doesn't work
I was wondering the same and maybe this code is related:

function refreshless_page_attachments(array &amp;$page) {
  $request = \Drupal::request();

  // Refreshless is only sensible when there is an actual session (otherwise the
  // entire page can be cached by the page_cache module, and be sent more
  // quickly than Refreshless could).
  // @see \Drupal\big_pipe\Render\Placeholder\BigPipeStrategy.
  $session_exists = \Drupal::service('session_configuration')-&gt;hasSession($request);

  $page['#cache']['contexts'][] = 'session.exists';
  if ($session_exists) {
    $page['#attached']['library'][] = 'refreshless/refreshless';
  }
}


It could be that for anonymous user the library is not loaded at all(check the comments in the function).

However I am not 100% sure.

Update: if you log in, you can see that refreshless.js is loaded.