It's not that they necessarily block out light (although some are large enough to), it's that they can, if left alone, choke out the roots of small plants. If left alone too long, something like bisquamata that forms a dense carpet could conceivably stunt the growth of a plant's crown or rhizome. At least, I would assume so.
I have a dedicated pot of U. bisquamata that's been growing unimpeded for nearly two years, and if I were to pop it out, it would be almost solid with stolons, and the surface is an impregnable mass of leaves.