Autoloads must be nodes - autoload stands for auto-loaded node/scene. I haven't checked if it has changed in recent releases, but at least as far as I know, you must use a class that extends node.

There's no important reason to make a scene for you. In the tutorial, I just use it to show how the autoload shows the same way as your scene in the remote scene tree. Also, you can use a scene to have an autoload that is more than a single node.