This is certainly a possibility. Water is a byproduct of lipolysis. Personally I have noticed that I seem to need less water when out for long days in the mountains. Since I’ve been coaching more and more mountain athletes I frequently hear that they also record much less water consumption that would be expected for their activity. Elite alpinists like David Goettler routinely does multi-hour, strenuous workouts on alpine terrain in Chamonix drinking 1/2 liter of water and one gel.
Another benefit for the highly fat adapted mountain athlete.