Get the latest comment from a custom post type where depth = 1?

Set the “parent” parameter as 0.

$args = array(
    'parent' => 0,    
    'post_type' => 'custom-post-type',
    'number' => '1',
    'orderby' => 'date',
    'order' => 'DESC'
  );

$comments = get_comments($args);
foreach($comments as $comment) :
    echo($comment->comment_author . '<br />' . $comment->comment_content);
endforeach;