I think that's waste of money, time and waste of zoo animals, to be released in the wild, when already there are orangutans on both Sumatra and Borneo, who are strugling to survive; The zoo animals are very valuable stock they shouldn't be released in the wild, unless the species in question is already extinct in the wild or it's wild population isn't self-sustainable and big conservation project is ahead to return the species in the wild, or if the captive population it's not self-sustainable and there is not possibility to bring in wild founders, as Frankfurt zoo will do or already did, with the South-central subspecies of black rhinoceroses.
Otherwise, zoo animals must be kept secure in the zoos and not wasted releasing them in the wild, when again, they are struggling a lot to survive day by day with all that poaching, deforestation, fires, etc.
Zoos should not feel guilty or obliged to release animals back in the wild, because they are already doing great job maintaining captive sustainable populations. Protecting wild animals in the wilderness is a major task for governments, not for zoos.
Also the Aspinal's zoos shouldn't release the gorillas in Africa, in my opinion.
The moto of zoo should be: breed wild animals and keep them secure in captivity, send them to other zoos for their population to expand; Only release them in exceptional circumstances.