"If you are having trouble using the lists, please contact email@example.com." - https://lists.archlinux.org/
I guess a more useful /dev/null address could be put there, as we can assume, that the default mailman@ mailing list is either deactivated or doesn't exist anymore. However, that contact mail address is more aimed at list administrators (AFAIK).
I'd read that page as saying mailman@ is for me as a non-list-administrator; like postmaster@, but Mailman specific. The `List administrators...' start to the previous paragraph, I take to apply only to that paragraph.
mailman@ has probably just been dropped by accident somewhere. I see lists.archlinux.org's Postfix is happy with `mailman' as a recipient, tested with `rcpt to:...', unlike a random username, so it clears the first hurdle.