They're definitely abusing the word 'most' there. I've stayed in easily 20 hostels at this point, and not once has a 'sleep sack' been required. And in all my hostel research, I have never stumbled across someone mentioning they needed one.
I don't think I have on the relatively limited occasions I've stayed in a hostel (or a mountain hut). That said, I'd certainly heard of sleep sacks sometimes being used/required in hostels. So it can't be all that rare--or maybe it used to be more common.
