How to sort a non-meta field in the User Admin Panel?

The WP_User_Query::parse_orderby() actually supports sorting by the number of posts, but because WP_User_Query::parse_orderby() limits by what users can be sorted, accomplishing a custom sort is a bit of a hack.

Here’s the workaround I’ve created (semi-tested):

add_filter( 'manage_users_sortable_columns', static function ( $columns ) {
    $columns['articles_count'] = array(
        'articles_count',
        false,
        __( 'Articles' ),
        __( 'Table ordered by number of articles.' ),
    );

    return $columns;
} );

/**
 * Because `parse_orderby()` will discard 'articles_count',
 * set it to `post_count`, and create a custom query var for
 * reference later.
 */
add_action( 'pre_get_users', static function ( $query ) {
    if ( 'articles_count' !== $query->get( 'orderby' ) ) {
        return;
    }

    $query->set( 'orderby', 'post_count' );
    $query->set( 'sort_articles_count', true );
} );

/**
 * Check for our custom query var, and change the SQL
 * for sorting users.
 */
add_action( 'pre_user_query', static function ( $query ) {
    if ( ! $query->get( 'sort_articles_count' ) ) {
        return;
    }

    $search  = get_posts_by_author_sql( 'post' );
    $replace = get_posts_by_author_sql( 'articles' );

    $query->query_from = str_replace( $search, $replace, $query->query_from );
} );