[Mailman-Developers] Postfix instructions in 2.0.10's INSTALL

[Dan Mick]

2.0.10's INSTALL file has this section:

    Problem:  I use Postfix for my MTA and the mail wrapper programs
              are logging complaints about the wrong GID.

    Solution: Create a separate aliases file for Postfix in its
              main.cf config file under the variable "alias_maps". Put
              the file somewhere in Mailman's home directory, or
              somewhere else where the user mailman has write access
              to it; *as user mailman* call Postfix's "postalias" on the
              alias file.

              % postalias <the alias file>

              Also as user mailman, run

              % python -c'import os; print os.getgid()'

              This should print out the group id that Mailman should
              be configured to expect when the mail wrapper programs
              are run.  Call it "thegid".  Rebuild Mailman with

              % ./configure --with-mail-gid=thegid

What is this supposed to do?  "postalias <a private alias file>"
appears to simply create an alias.dir/alias.pag pair in that
same directory, but not affect the Postfix delivery at all.
Is this a Postfix version skew problem?  Is it expected
that postalias actually "installs" the aliases somehow?
It doesn't seem like that's the case.

(There was a question on mailman-users that prompted me
to wonder what this meant.)