I'm sure they have done their research, but you might expect the tapirs to experience high levels of stress, as the olfactory stimulation of the scent of the tigers would be great enrichment but they'd have no prospect of avoidance, so unless they became completely desensitized to it, they be constantly exposed to the threat of danger. At least the orangs could climb to points in the enclosure where they might feel safe?
Having seen the Dama gazelles at Marwell casually sniffing at the chain link fence dividing them from the cheetahs without a single nervous movement while the cheetahs, centimetres away, appear to be excited/frustrated, maybe prey species in zoos are quite capable of judging the integrity of the barriers that protect them from nearby carnivores and it is this that governs their sense of threat, rather than the constant proximity.