Function Reference/wp list comments
'end-callback'
( callback ) The name of a custom function to use to close each comment. Using this will make your custom function get called to at the end of each comment, bypassing the WordPress default of using </div>, </ol>, or </li> based on the style parameter. Use to customize the ending tags for a comment. The callback is separate from the end-callback to facilitate hierarchical comments. Use with caution.
Ou seja, o end-callback padrão adiciona o fechamento da lista (que pode ser </div>, </ol>, or </li>). Basta adicionar o <hr /> em conjunto com o fechamento.
Detalhe, o parâmetro 'comment-body' não existe.