For practical purposes, I agree with the original author that there are things that can be done on the desktop that can't be done on the browser, but not the other way around. This is just a simple extension of the fact that the user has the same computer and the same internet connection, but in a browser-based app, you don't have access to all the computer's resources. Therefore, you have less overall power available and there are things that theoretically can't be done. If you have unlimited bandwidth and transfer speeds, then there is no difference.

And there's nothing you couldn't do with a desktop app, if for no other reason that you could "Greenspun" a full browser into your desktop app to include anything that theoretically couldn't be done without a browser.