MailPoet 2.X - Custom shortcodes - How to retrieve normal WP user data?

custom-shortcode-wp-user.php

<?php

/**
 * 
 * @param string $tag_value
 * @param int $user_id mailpoet's subscriber id
 * @return type
 */
function mailpoet_shortcodes_custom_filter( $tag_value , $user_id ) {
    
    // run this code only for my custom tag [custom:my_custom_tag]
    if ($tag_value === 'my_custom_tag') {
        // get the wpuser_id of that subscriber
        $model_user = WYSIJA::get('user','model');
        $subscriber_data = (array)$model_user->getOne(array( 'wpuser_id' ) , array( 'user_id' => $user_id));
        
        // get the wp-user data based on the wp-user id
        $wpuser_data = get_userdata( $subscriber_data['wpuser_id'] );
        
        // return the key you want
        $replacement = $wpuser_data->last_name;
    }

    if ($tag_value === 'blog_name') {
        $replacement = get_bloginfo('name');
    }

    return $replacement;

}

// filter registration allowing the two parameters $tag_value and $user_id to be passed
add_filter('wysija_shortcodes', 'mailpoet_shortcodes_custom_filter', 10 , 2);

I understand the approach, so that's good. However, I am having a problem within the shortcode establishing the '$user_id' value that is being used in the SQL WHERE clause. Does it come into the shortcode as a parameter value, or is it established in some other manner?
Also, is it correct to make a distinction between '$user_id' and 'subscriber id'?
I have tried many things to establish either the $user_id value or the subscriber value within the shortcode, but have not been able to make it work.