Thanks for writing in!
As you mentioned Cornerstone is included with X, similarly there is no need for separate validation for the cornerstone.
If you have validated the theme no need to install or validate cornerstone separately.
Please check validation guide for clarification
If you still facing the issue, please send us your website details so that we can have a look.
Please provide following information:
Set it as Secure Note
* Link to your site
* WordPress Admin username / password
All the best!