> On Debian GNU/Linux this file is a symbolic link pointing to /var/run. The contents of this file are regenerated upon every system boot based on the contents of /etc/motd.tail.
The fundamental problem here is the blog author didn't realize that `echo foo > /etc/motd` wouldn't work, even though the manpage did, in fact, inform him that /etc/motd was just a symlink to something that gets regenerated at boot time. I'm not really sure what the manpage could have done to make it more obvious what his problem was.
Now, it's possible the manpage is out of date, and that it's regenerated via some other mechanism than /etc/motd.tail. I don't know (since I don't use Debian, or even Linux in general). But the salient point here was that it points to a file that gets regenerated at boot time.
*Edit: On second look, it sounds like this change makes it regenerate on each login, not just each boot. I can understand why someone might think that `echo foo > /etc/motd` followed by a login would then print 'foo', given the current manpage. But they certainly wouldn't expect that it would still print 'foo' after a reboot unless the break the symlink (and breaking the symlink fixes the author's complaint).