We do not authenticate to Google. That’s just a bug in the Drupal Discourse SSO plugin that I haven’t been able to fix. Just click through to the forums after you get the message, it’ll work fine.
You are right.
Something similar happens when I click the Login button on the top of the forum page, I get
Access denied. You may need to login below or register to access this page.
Below that a choice, and below that my login data prefilled.
Thanks for all your work, presence and openness.