That is a good idea with only one drawback; it isn't feasible in most terrain. You rarely, if ever, encounter a situation where the rim is readily accessible at the spot where an emergency occurs. In a flash flood, the "rescuers" perhaps would have been aware of the forecast - where would they have lowered their ropes, assuming they had enough rope (rather unlikely).
Even then, you have one rope and seven in the party to climb out. This takes time, which is what you don't have in a flash flood situation. None of the discussions of canyoneering equipment mention ascending equipment, necessary if you are going to use the rope.
My question about their rather odd late start was answered by today's article in the LA Times - they had taken a five hour class in the morning, hence their afternoon trip in what is evidently regarded as a basic, beginning excursion. They were just quick enough to miss the upgraded alerts issued by the weather folks.
Here is an article which portrays the upside of canyoneering, leaving out the nasty bits concerning flash floods -
http://articles.latimes.com/2013/may/13/travel/la-tr-utah-canyoneering-20130513 The basic issue I have with canyoneering is that it involves rappelling - an activity which does not require particular strength or agility since gravity is your friend. Rappelling does have hazards and statistically, is just as dangerous as leading a pitch in rock climbing. Like any activity, approach canyoneering with an understanding of the potential hazards and thee necessary safeguards, including the timing of your enterprise.
I can only quote the French mountaineer Gaston Rebuffat -"You will never regret beginning too early, you will always regret leaving too late" - especially when there are clouds in the sky and rain in the forecast and you are planning a trip through one of Nature's funnel systems (watersheds).