Probably just history. That code dates from c. 2001 or even earlier,
before acs-mail-lite existed.
However, there are lots of calls to ns_sendmail throughout
the toolkit, as grep will show:
find . -name "*.tcl" -print | xargs grep ns_sendmail
So, you probably need to configure a basic SMTP server on the box,
and/or drop in a replacement ns_sendmail proc which does what you
want.
Why doesn't your box have a local SMTP server listening on localhost
anyway? AFAIK it should have one.