Lots of people saying 'Old news', of course it is, but that does beg the question of how so many sites still get it so badly wrong?
One genuine failing in the article is that it doesn't even mention the right solution, which is to use a Robots meta tag within resources you want to hide like
<META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW">
So the file will be ignored IF a spider finds it, but won't be advertised via the robots.txt file, this only works for HTML resources though.
The bigger 'take away', of course, is the fact that you should never rely on obscurity as your only security; if you don't want files to be accessible; block/control access on the server-side.