Thanks - I've just added a paragraph based on this feedback to the relevant page on updraftplus.com:
Also – note that it’s possible for DISABLE_WP_CRON to be set in a file other than wp-config.php; wp-config.php is simply the most likely (99%) place. If you have a warning about DISABLE_WP_CRON, but it is not found in wp-config.php, then it will be somewhere else – you will need to hunt for it.
If you feel very skeptical that DISABLE_WP_CRON is set at all, then go into the expert/debug tab for UpdraftPlus, and click on the 'phpinfo' link. That will show the settings of all constants. But basically, it must be set somewhere, because the warning you're seeing does one thing only: says "Hey, PHP, is DISABLE_WP_CRON set?", and then displays the warning.