CiviMail does not use the editor set in 'Display Preferences'
You didn't mention the path to the config.js that you're editing, which I think will help people answer your question.

However, my guess is that you are installing Civi using the Roundearth plugin, because that would account for you having two different config.js files. I'm also guessing you're editing the file at &lt;projectroot&gt;/vendor/civicrm/civicrm-core/bower_components/ckeditor.  There's a second config.js at &lt;projectroot&gt;/web/libraries/civicrm/bower_components/ckeditor.  Hopefully this is enough to help you solve your problem.