Fake post excerpt

// Remove the editor. Change 'post' to your custom post type.
add_action( 'init', function () {
    remove_post_type_support( 'book', 'excerpt' ); // also tried to remove the post_content editor for the CPT, but it still shows
} );
add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
// was not sure if the following 3 lines were necessary so commented them out, did not work either way
    // Get the current post content and set as the default value for the wysiwyg field.
    // $post_id = filter_input( INPUT_GET, 'book', FILTER_SANITIZE_NUMBER_INT );
   //  $post_content = get_post_field( 'post_excerpt', $post_id );
    $meta_boxes[] = [
        'title'      => 'Excerpt for Book post type', // this is the Field Group Title
        'post_types' => 'book', // Change 'post' to your custom post type.- done
        'fields'     => [
            // Register a wysiwyg field of which the content is saved as post content.
            [
                'type' => 'wysiwyg',
                'id'   => 'excerpt', // This is the must!
                'name' => 'Reason for snippet', // label for the custom field
            ],
            // Custom style to overwrite the editor style set by WordPress.
            [
                'type' => 'custom_html',
                'std'  => '<style>#wp-content-editor-tools{background:none;padding-top:0;}</style>',
            ],
        ],
    ];
    return $meta_boxes;
} );
// Set the value for the 'excerpt' field.
add_filter( 'rwmb_content_field_meta', function() {
    $post_id = filter_input( INPUT_GET, 'book', FILTER_SANITIZE_NUMBER_INT ); //assumed I should use book the CPT instead of 'post' here
    return get_post_field( 'post_excerpt', $post_id ); // assumed I should use 'post_excerpt instead of 'post_content here
} );
// did not think the following were necessary in this case so commented them out
// Do not save 'content' field to post meta.
add_filter( 'rwmb_content_value', '__return_empty_string' );