dashboard_activity hook

Using the dashboard_recent_posts_query_args filter, you can add your custom post types to the query args, and they’ll be included (untested):

add_filter( 'dashboard_recent_posts_query_args', static function ( $args ) {
    if ( ! is_array( $args['post_type'] ) ) {
        $args['post_type'] = array( $args['post_type'] );
    }

    $args['post_type'][] = 'cpt';

    return $args;
} );