Consider following..
robots.txt doesn't block SE in the way that they can't see beyond it, it is for informational purposes only, it is actually up to SE if it want to obey it or not.
.htacces file does block SE from physical access to the certain directories, but can be indirectly detected by response headers that in the rules
directory name of any plugin can be simply renamed with the little tweaking
there is more plugins out there can do autoposting, not just WP-Robot
Therefore it should be pointless for G or any other big SE to develop any specific mechanisms that look into WP plugins directory and make any assumptions based upon it. They already have mechanisms on the place to detect duplicate content, all you need is just to spin it better. However it is always good idea to restrict access to whole wp-content directory with .htaccess but it does require some tweaking as it may block some plugins and/or themes