The team achieved its feat using guide RNA that steers the editing enzyme toward a matching sequence of target RNA molecules. In the lab, it's highly effective -- it fixed nearly all of the broken RNA targets in muscle cells, leaving cells that were far healthier.
Of course, this is in lab conditions. The scientists readily admit that there's much more work to be done before gene editing could help in the field. More than anything, it would be difficult to send RCas9 to patient cells. The benign viruses you'd normally use for gene therapy just can't hold a full-fledged Cas9 payload; the researchers had to prune theirs to make it fit. And of course, there's the question of conducting trials and proving that the technique is safe.
Nonetheless, this could prove to be hugely important for treating these RNA diseases. Right now, there aren't any therapies that would stall the onset of diseases like ALS. This wouldn't likely represent a cure, but it could give patients full functionality for longer before the effects of their diseases set in.