The textarea right underneath this inexplicably shameful text

#!!# These ACLs are crudely constructed from Exim 3 options.
#!!# They are almost certainly not optimal. You should study
#!!# them and rewrite as necessary.

begin acl

That's the very next place they offer to make any changes. I just stuck the host_accept_relay = localhost line right in there.

(Really, who would make software that comments on itself, "this is really crappy, you should change it"?)