Yes, that is right. A new code is generated every 5 minutes for the download window. It is checked every time someone clicks the download link, and if 5 minutes have passed, you won't be able to download. I made this protection so that people can't direct link to the download page.
Anyhow, just keep the actual game pages open, and then when you are ready to download, click get it, then download, and it will work fine