How do I set a Boolean config variable to false?
When setting a boolean with drush, you need to use 0, not false.
pk@DC:~/$ lando drush config-get advagg.settings enabled
'advagg.settings:enabled': true

pk@DC:~/$ lando drush config-set advagg.settings enabled 0 -y

 // Do you want to update enabled key in advagg.settings config?: yes.                                                  

pk@DC:~/$ lando drush config-get advagg.settings enabled
'advagg.settings:enabled': false