Endemic to X means that it's geographical distribution is confined to X. The (original) geographical distribution of basically all bird species is fairly well known, due to the efforts of 19th and 20th century scientists and their collection of museum specimens. The reference work here is the "Peters check list", completed over the course of several decades in the 20C by various ornithologists.
Of course, you have to check when they say "species" whether they're referring to a subspecies "elevated to species status" by dodgy "phylogeography" genetics studies.
The article says they found 18 at 3 different sites, in a region of Congo that had been dangerous for the public to visit but had recently become safer. That does not prove a healthy population but it seems a step up from “last survivor”
