Hello,
This guide has incorrect information. It says $post_data is “The array containing the post data to be filtered. This array is in the format used by the WP function wp_insert_post($post_data)”
However, the wp_insert_post format puts meta fields in ‘input_meta’. The $post_data field here puts it in ‘post_custom_fields’.
Could you please note the full format in the documentation?