Does it? The linked blog post makes no mention of the preference setting you describe, so the author was probably running without that setting (which is off by default). I'll have to try that when I upgrade to ML next week.
Before displaying the Save As dialog for naming your new file a write to the original takes place. They probably just do a copy and rename file operation afterwards.
Interesting fact: if you abort the Save As dialog and try to close the original they ask you to "Revert Changes".
The old model was revert-by-default. You had to manually save your changes.
The new model is save-by-default. You have to manually revert your changes.
In the "File" -> "Revert To..." menu you have multiple options, such as revert to last saved document and revert to last opened document. These persist even after restarting the application.
Then OS X is not supposed to overwrite your file without your consent, but it does in this case.