Interesting - I've had a plenty of nights of drink, including some where I felt unwell the next day, but never had either of those symptoms. I have, however, woken up recognizably dehydrated a couple of times.
I wonder whether some of the "myth" comes from the experiences of people like me, accurately reported but for some reason not defined as hangovers by the researchers behind the study cited.
I've mistaken caffeine withdrawal for a hangover before, partly because of similarity in symptoms (headache; nausea; photosensitivity) and partly because it tends to show up around the same time (Sunday morning). This may account for the popularity of coffee as an alleged hangover cure.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.