I do not recommend people follow this article. I do not recommend postfix_to_mailman.py which is a 3rd party module neither distributed nor officially supported by the GNU Mailman project. I also don't understand what is lacking in the official installation manual ("the documentation currently available" link at the beginning of this article, and I don't understand why there would be anything in even these instructions that would prevent them working with lists created via the web interface as long as the permissions are correct on Mailman's data/aliases and data/virtual-mailman files which aren't even used by postfix_to_mailman.py anyway.