Who knows what that kayaker saw?
In flight from behind, the Ivorybill has large white wing patches, and that is what is apparent in the video. The Pilaeated does not.
I agree that they look alike, but if you are just an avid bird watcher, you instinctively know what you need to be looking at right off the bat.
And to be honest, those differences to my eye are VERY noticeable. And I'm no scientist. Bird watching is strictly a hobby.