My view queries a post type ("resales") and I want visitors to be able to use the parametric search filter to select posts by authors with different roles, so the select dropdown would need to display the user roles that have authored those posts.
Unfortunately this is not possible even though the link you provided has a similar solution but the exact solution for this would require alot more logic.
This is because with the author role filter you will need to join the Roles table and the Author information before comparing it to the posts.
We currently have an author filter for this but to filter by the authors role will be more difficult as such this would fall out of our support scope and I would recommend contacting one of our certified partners in order to get this done.