OK, the option is not missing, I was using it wrong. However, it still doesn't do anything!
$ mailman -C /etc/mailman.cfg start --force usage: mailman [-h] [-v] [-C CONFIG] {info,status,help,withlist,start,unshunt,create,stop,remove,lists,reopen,version,import21,inject,members,qfile,restart,aliases} ... mailman: error: A previous run of GNU Mailman did not exit cleanly. Try using --force.