If the WordPress user’s Discourse Username is not set, posts will be published under the username you have set for the Publishing Username on the plugin’s Connection options tab. To have all posts published by Simple Steve, create a Discourse admin user with the name Simple Steve (username simple_steve) and enter that username into the Publishing Username field on the Connection options tab. You then need to make sure that none of the authors on your site have set their Discourse Username. The Discourse Username is set on the WordPress user profile page.
When your authors publish a post, they will see a warning that they have not set their Discourse Username. They can ignore this warning.
Since you are using SSO, you will need to be sure that the SSO Provider setting Create or Sync Discourse Users on Login is not enabled. When this setting is enabled, the Discourse Username field is automatically filled in by the plugin when users login to WordPress.
There are a few small changes that could be made to the plugin that would make this process a lot more straightforward. I’ll look into doing that.