Author list based on recently active

I would take a different approach to this using a direct database query to get a list of author IDs ordered by recent posts.

global $wpdb;
$user_ids = $wpdb->get_results(
    "
    SELECT DISTINCT post_author
    FROM $wpdb->posts
    ORDER BY post_date DESC 
    "
);
if ( $user_ids ) {
    foreach ( $user_ids as $user_id ) {
        $user = get_user_by( 'id', $user_id );
        // do stuff with $user
    }
}