OWA login with e-mail address

How to Configure Forms-Based Authentication for Outlook Web Access that when configuring a CAS server with forms-based authentication, and selecting "User Principal Name" as the sign-in format, that, "Users can access Outlook Web App by entering their primary e-mail address or by entering their UPN."

However, I'm finding that users can only log in using their UPN value. When I change a user with UPN value 12345@company.com to have a primary e-mail address of joe@subsidiary.com he's still only able to log into OWA as 12345@company.com .

Is this a bug in the documentation, or in my implementation, or am I not understanding what is meant by the sentence in the linked doc?

Thanks,
- Jim