The doing_wp_cron parameter is only used in conjunction with the ALTERNATE_WP_CRON
option. Plesk doesn't use the ALTERNATE_WP_CRON
option, rather it disables the build-in WP cron all together with the DISABLE_WP_CRON
option and instead calls the wp-cron.php page directly via a scheduled task (cron job). Which is, in most cases, the most reliable and efficient way the handle the WP cron.