This assumption is absurd, of course. Soft drinks, coffee and other high-sugar beverages actually have a dehydrating effect in the body. They cause the excretion of more water than they provide, resulting in a net loss of water, not a net gain. This is due to the high sugar and high phosphate content of soft drinks. In coffee, it's due to the caffeine and added sugars. Long-term consumption of these beverages leads to a state of chronic dehydration that is often misdiagnosed as asthma or other diseases. The book to read on this is called Water: For Health, For Healing, For Life.
The National Academies apparently hasn't read this book, nor are they aware of the fundamentals of hydration and nutrition. To categorize the water content of soft drinks as quality hydration is the sort of oversight that might be acceptable by a student in a high school science fair, but it's appalling to see it from an organization that purports to be so well informed. In reality, it's just bad science, plain and simple. Or just outright ignorance.
The report set general recommendations for water intake based on
detailed national data, which showed that women who appear to be
adequately hydrated consume an average of approximately 2.7 liters (91
ounces) of total water -- from all beverages and foods -- each day, and
men average approximately 3.7 liters (125 ounces) daily.
Moreover, we concluded that on a daily basis, people get adequate
amounts of water from normal drinking behavior -- consumption of
beverages at meals and in other social situations -- and by letting
their thirst guide them."
Regarding salt, healthy 19- to 50-year-old adults should consume 1.5
grams of sodium and 2.3 grams of chloride each day -- or 3.8 grams of
salt -- to replace the amount lost daily on average through sweat and to
achieve a diet that provides sufficient amounts of other essential
nutrients.