ALTERNATE_WP_CRON is defined in the plugin itself. The plugin checks to see if it is defined either way and if not, it defaults it to true.
The issue is not so much whether or not you have it defined in wp-config because the plugin itself defines it.
Lines 281 – 283 in /wp-content/plugins/all-in-one-event-calendar/all-in-one-event-calendar.php:
// Use alternative cron Version to bypass problems in wp_remot_* calls
if( ! defined( 'ALTERNATE_WP_CRON' ) ) {
define( 'ALTERNATE_WP_CRON', true );
}
This is the reason why if you set define it to false elsewhere, it removes that from your URLs.