You're using macports, homebrew or fink. If the command is in /usr/local/bin then "its not in OSX" stands as a problem. Its not in $PATH for a native install without adjunct s/w
All of this can be avoided if nmh adopts "install package x" requirements because groff does handle .fc.
The point is, (I believe) Ken is (rightly) trying to minimise the surface of required dependencies to install, to ones in the base distro of most Unixen.
-G