Irreducto ad absurdium
"This sticking point appears to be whether water alone, and how much, will cure dehydration."
Er... that's not what the Brussels judgement said; it ruled that some the claimed associated benefits of hydration being made were too vague. There's no question of whether water "cures" dehydration!
A prof on the radio made the point that almost everything edible aides against dehydration, as almost all drinks and foodstuffs contain water - technically if this ruling went the other way, even beer manufacturers could sell their product as health giving based on its water content.