Hi,
Well, there is no option for it, but a small custom code snippet may do the trick for you.
Add this custom code to the functions.php in your theme/child theme directory (copy from line 3 only!). Before editing, please make sure to have a full site back-up just in case! Let me know if you need help with that.
Make sure to save the plugin options after this, to clear the cache as well.
This should replace the author names with the display name, as it appears in wordpress posts.