It tells you the author, though, which is enough to find it. Here is the abstract: Effect of Sodium Supplements and Climate on Dysnatremia During Ultramarathon Running.
Key takeaway from the abstract: “avoidance of overhydration” helps prevent exercise-associated hyponatremia, and “avoidance of dehydration” helps prevent hypernatremia. Well… okay. What that really means is that on average, people’s hydration strategies are all over the map, and the level of hypernatremia or hyponatremia they end up with is also all over the map.
However, if we assume that a) yes, hydration strategies are all over the map and b) hydration requirements are all over the map, this is exactly the result that we would expect. Some people who actually don’t have issues with losing too much water or sodium to sweat are still going to overdo it, and some people who do have issues aren’t going to do enough - as well as every other combination.
I’d be interested in a study like this that attempted interventions in a non-control group - did some testing of runners’ sweat rates and sweat sodium concentrations, suggested a hydration strategy, and then observed whether individualized hydration strategies yielded better final hydration results than “all over the map.”