Thanks for explaining. The mailman2discourse script does not play a role in this because it does not import the messages. The import of the messages is done via the Discourse mbox importer.
I think it would set the author of all mails to listname@example.com. I do not see anything in discourse/script/import_scripts/mbox/importer.rb that would suggest otherwise. It would be a feature to add and maybe control via a use_reply_to_instead_of_from field in the settings.yml file?