This is a basic example of using the elastio-asset-account-stack-set terraform module with the service-managed AWS Cloudformation StackSet.
You'll need to deploy it from the AWS Management account. You'll also need to specify both the input variables: accounts and organizational_unit_ids.
AWS API requires at least one org unit ID that contains the provided accounts. It doesn't mean you'll deploy the StackSet into the entire org unit, it's just a quirk of the AWS API. The Stack set instances will still be deployed into the accounts specified in accounts.
If you want to deploy into the entire org unit, then modify the deployment_targets as needed for your use case.