That would let him receive emails for himself on mailing list mode, rather than force it for all users.
Maybe he has enabled mailing list mode but not set it for all users.
Incidentally, the first two settings below are confusing.
default email mailing list mode
Send an email for every new post by default.
default email mailing list mode frequency
Send an email for every new post
Users who enable mailing list mode will receive emails this often by default.
disable mailing list mode
Disallow users from enabling mailing list mode (prevents any mailing list emails from being sent.)