I disagree. Wabi kusa means chaos in nature in Japanese. Wabi kusa isn't specifically limited to balls of plants. It is supposed to have imperfections, which makes it look natural.
The literal definition, yes (which doesn't actually exist, it's more of a paraphrasing). But the definition in the hobby of Wabi Kusa is a ball of emmersed grown plants. I know where you're coming from, as I thought the same thing not too long ago. Do some looking around, though, and you'll find what I'm saying is true