It seems like a tremendously difficult task to declare that a particular individual is an endling. Do you know how a zoologist would infer such a thing?
Only a guess, but if there is no sighting of it in the wild for a very long time (eg. from the article it was 10 years, but I guess it depends on species, how long does it live, how many offsprings it produces etc) and there is one living in captivity it is not that hard to declare.