Without going into lengthy discussion on how good zoos are with ex-situ conservation, this "only one-third of species are threatened" is extremely simplistic. I assume they looked up the IUCN status (which is fairly conservative, if you ask me) for the species they had and then calculated this number. This is easy to do but doesn't do the situation justice.
For example, Cinereous Vulture (NT), Bearded Vulture (NT) and Griffon Vulture (LC) are not endangered according to the IUCN. Yet the European populations (especially in the Alps) are highly depleted and captive breeding is a tool now used to raise population numbers, with quite some succes. Despite not deembed threatened by the IUCN, they very much benefit from ex-situ conservation. Other examples of such species (with varying degrees of succes) are Grey Partridge (LC, UK), Eurasian Harvest Mouse (LC, UK), Corn Crake (LC, UK), Sand Lizard (LC, UK), Fire Salamander (LC, NL), Common Spadefoot (LC, NL), Garden Dormouse (NT, NL), European Hamster (LC, NL), European Otter (NT, NL), Ferruginous Duck (NT, Germany) and European Golden Plover (LC, Germany).