He probably did not know about a hydration system, or if he did, forgot to mention it.
Nobody can survive long once your dehydrated, and having a sack, of whatever capacity, with a built in hydration system, is a more than useful facility.
My friends wife, borrowed my Cloud breaker, three years ago, whilst doing the Three Peaks, in August of 2014.
She did them in almost heatwave conditions, and commented on just how beneficial having my sack was, especially on Scaffel Pike.
Her photographs from the summit of Ben Nevis, were remarkable for the clarity, and i was slightly envious that she had such magnificent weather.
Carrying a separate water bottle is alway's a nuisance, as you have to physically reach for it every time, thats why a Hydration sack is almost an essential bit of it, as you can sip water every few minutes, staying hydrated at all times.