The dates are good, but the case is not truly made. As I mentioned a couple weeks ago, the rocks and sediments of the region match a different scenario, with the canyon not being cut down by a river but dug from underneath by groundwater. The thing is, both scenarios could produce the same set of cavestone ages. That's because those ages represent one thing: the elevation of the water table when the deposits formed. To get from there to the downcutting of a canyon, as Polyak proposes, is a leap unless there is other evidence of an actual canyon.
Of all the coverage in the media, only Larry O'Hanlon's story for Discovery News makes this distinction. And for good measure, he cites Carol Hill's paper in Geomorphology proposing a novel alternative scenario that accepts Polyak's downcutting theory but invokes the groundwater-sapping mechanism farther upstream, near the Kaibab Uplift. One way or another, we have a new ballgame in Grand Canyon studies.