Hows about this:
If you run /usr/lib/mailman/bin/genaliases you should find the files aliases and aliases.db are populated correctly in /etc/mailman.
I got an error when I ran this as /usr/lib/mailman/Mailman/mm_cfg.py has an empty DEFAULT_SERVER_LANGUAGE on line 76.
I set DEFAULT_SERVER_LANGUAGE = 'en' and the script ran producing the above files which will resolve the errors seen.
However, this appears to insert similar aliases as described in the howto when editing /etc/aliases.
Which, in turn, begs the question, should that step be reworked instead of altering aliases file running genaliases instead? At this point think we should refer to the developers for some input as to the interaction between mailman and the ispconfig configuration scripts.