I read about this and am an avid Zion visitor. All were experienced canyoneers and received their permit at 7:30 AM and a flash flood warning at 2pm. They went into keyhole at 4pm and the skies unloaded 1/2" in less than an hour into very tight canyons that flood completely in minutes. Under those conditions they had no chance. Those amazing slot canyons were cut by flash flooding over thousands of years and the process continues.
https://www.youtube.com/watch?v=RYBtEBSu1n8
Sorry for your loss and condolences to their families Hugh. One was from my town but I did not know him, although we may have shared a few nearby trails over the years.