I’m not sure about that. The WP Discourse plugin marks user’s email addresses as valid if they respond to the new user notification email that is sent by WordPress to a user when they first register on a site. A lot of plugins override this functionality. If you run into problems with existing Discourse users not being able to login after you enable SSO, post about it here and we’ll sort it out.
For a general overview of the issue, see Debugging and fixing common SSO issues.