My studies of many stories of water intoxication tells me that the problem of drinking this amount of water in a short period only affects that portion of the population who have a reduced electrolyte level or salts in the first place.
If you go swimming in fresh water, your hands and feet will get wrinkly from water "drawing" the salt out of your body thru the large pores in your fingers, palm and the soles of your feet. This goes away in a short time as the body replaces the salt in your hands and feet. This will not happen in salty water normally.
The problem of drinking too much water at one time would also be the same happening here.
The water taken in, is distributed in the body too fast into the brain, resulting in a fast swelling and creation of cranial overpressure, resulting in the loss of blood circulation and leading to the death of your brain cells from within.
This would also explain the lack of brain activity in this tragic story.
In my survival trainings, I always expressed the importance of not drinking more than a single glass
(4 ounces/200 ml) of anything at one time and keeping it in your mouth for a bit to "pre-warm" it so the body does not consume the extra calories needed to "heat" the drink to body temperatures in the stomach. Especially in winter, this steals calories needed for survival.
Just in the same way you should never eat snow or ice (besides dirtiness), the body must use lots of calories to re-heat the snow to body level temps and burns needed energy.
All of the needed intake of fluids must be distributed over the day and while, working at moderate temps.
Also, coffee and tea are now considered "water" intake contrary to older teachings. BUT, juices, wine and milk are not so-called "water" intake. They are too concentrated and need water to facilitate digestion. They do taste good and relieve the body of fluid needs but are not WATER.