Maybe.  But if so, they probably would not have drowned while wearing WW PFDs had they not become hypothermic first.  There's a reason the Sixmile outfitters make you don a drysuit and practice swimming before they let you do the Class IV-V run with them.  They want to know that you are comfortable in cold whitewater and have the skills to self-rescue if need be.  Based on the story, it's not clear that these two women had such skills.