Does the "downloader" install the agent software or is that only when you run the game installer?
I think they changed the file that is linked from your account page now that the game is live. There were (now only google cached) instructions on battle net to download the "Diablo-III-8370-enUS-Installer-downloader.exe", which is what is linked in the OP. This .exe downloads the 8gb of content into a folder of your choice. HOWEVER, the file that is on the account page NOW is "Diablo-III-Setup-enUS.exe". The setup.exe is about 40mb and when you run it, it goes straight to downloading and installing.
In any case, I got what I needed, so thanks again.