That is how it works now. When he says "it silently saves the original file", he's missing the point that OS X is always saving your files.
In Lion/ML, if you create a new file, Save it with a name, and afterwards add some text and then close the window, it doesn't prompt you to "save changes", since it's always saving changes. In a model like that, there's no reason for "Save As" to suddenly not save changes in the original window.
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.
In Lion/ML, if you create a new file, Save it with a name, and afterwards add some text and then close the window, it doesn't prompt you to "save changes", since it's always saving changes. In a model like that, there's no reason for "Save As" to suddenly not save changes in the original window.