That article is incorrect. Sorry to be so blunt but there is incorrect advice given there.
This is a document level URI...
This is a directory level URI...
Years ago, the advice was to put everything at a directory level URI. That was before we all started discussing Content Negotiation, Extensionless URIs, etc.
Use /directory/ for directory level and use /document for document level. Don't be misled into the whole trailing slash myth. Go extensionless and serve your documents from their appropriate directory.
There are plenty of proactive options you can take to make sure the above works 100%.
Are Yahoo! and Bing still stripping the trailing slashes from Display URIs? ;)