Likely the difference comes from the differing views of peole keeping different fish and tanks. I came in from the side where cichlids are the main interest. That group has a different need for dithers than a group where the fish are smaller. With cichlids it is often necessary to have a way to spread the chasing around as the fish of main interest begins to spawn. Some African cichlids will breed with just about anything that is in the tank, when one female becomes "ripe". If there isn't some way to distract the guys in the chase, one female can get beat up too badly. That's when a dither can be handy. It isn't so much that they do anything but they are just there and the chasing fish finds himself chasing something really quick and hardy that he can't catch. Meanwhile the female slips under a rock and rests for a bit.
Seems like your last item is closer to a good description of "dithers". They are not necessarily large nor small but fish chosen to stand around and "dither" until needed!