If you are an Atlassian partner, or your organization is a qualifying academic or charity body, you can apply for a free draw.io for Confluence Cloud license.
To do this file a support ticket (via email to email@example.com) detailing your organization's status and the URL of your Confluence instance (abc.atlassian.net).
Assuming your org is a valid case, we will inform you when your free license is setup. In the draw.io manage add-on admin panel, you need to stop any trial or remove any license, otherwise you will be charged.
The add-on will display as unlicensed, that is correct and the add-on will work properly as long as your domain is set to free on our systems.