Well, nuts. I read the press release, did some follow-up, and still managed to make a mistake in this morning’s post!
The outer ring is probably caused by Mab. But instead of being formed in a collision, it appears to be more likely that the ring is due to continuous bombardment of meteoroids on Mab, which dislodge material and forms the ring. Some of that material is swept back up, but the two processes balance. It’s like a sink which has the drain open but water coming in from the spigot; the water level stays the same, even though there is water flowing in and out of the sink.
Where an old collision plays a role is in the inner ring, discovered recently as well. That one has no obvious supply of dust, but the ring doesn’t look like it can last long (maybe a million years). So that is where there must by smaller moonlets, maybe a mile or so across, which are the source of dust through collisions.
My mistake in the earlier post was to conflate these two things. I apologize. I hope this doesn’t confuse people too much.
If you want more info, try the SETI page which has more details.