How to get all comment authors of a single post?

Writing a loop and going through every comment is the option that I can offer you. Take a look at this example:

function get_comment_authors($post_ID){
    // Get every comment on this post
    $args = array(
        'post_id' => $post_ID
    );
    $comment_objects = get_comments( $args );
    // Define an empty array to store the comments
    $author_array = array();
    // Run a loop through every comment object
    foreach ( $comment_objects as $comment ) {
        // Get the comment's author
        $author_array[] = $comment->comment_author;
    }
    // Return the authors as an array
    return $author_array;
}

So, get_comment_authors(123); will return every author that left a comment on the post that has an ID of 123.