The latest, greatest Ursack withstood an hour's worth of bear mauling without giving anything to the bear. Of course I wouldn't want mine chewed on for an hour -- the supplies inside would be unusable. But then if I am going into real bear country, I take the aluminum liner that is made for it.
As for the Whitney area, the bears only bother hikers in the parking area and campground, where the bear boxes are. Higher up, it is marmots and mice, not bears. So the sack would hold up just fine. I would hang mine off the side of a big boulder to discourage the little critters.
The Inyo page you linked to is the same one I used above. Note that they do not specify which brands or models are approved or rejected. All they say is "the containers must be designed to prevent access by bears". Ursack fits that requirement.
