Read the primary source rather than the write-up and the two do not agree, so here is what is actually in it.
Two things drive this: people drink less because drinking makes them feel full, and they restrict salt at the same time as calories without noticing. The result reads as fatigue, headache, cramp and light-headedness on standing, all of which get blamed on the drug. Fluid plus sodium fixes most of it within a day, which is also how you can tell that is what it was.
Where I think it is weakest: the comparator does most of the work in how this gets reported, and it is not the comparator most people think they are citing.
The bit I cannot resolve on my own is why hydration goes wrong on this specifically, given that nobody is telling us to drink less. Numbers rather than impressions, if you have them.
Note on sourcing:
Figures above are from the primary publication rather than the press summary. If a number here disagrees with one you have, post yours and we will work out which of us is reading a secondary source.