Yes, in that they are visual representations of where your mind runs to from a thought or belief.
It is also called "Spider Writing" because it often ends up looking like a spider: central theme then branching off into each subsection of where the mind runs (worries, ruminates etc) and then perhaps where it spirals off from each sub section.
I find this a very very fast way of getting to the root of things when needed!
You are indeed right that it is very similar to mind mapping but mind mapping is used more for problem solving and research in my experience. With cluster and spider writing you aren't solving a problem or digging/researching. You're just offloading all the machinations of the mind and having an awareness of how one thought can spiral into a whole bunch of (most often) useless drivel.
Have fun!