The Hacker News headline is misleading. The article calls it the "last major primeval forest in Europe" which is more accurate. Białowieża forest is around 40 000 hectares, while very few of the other ones left is bigger than 2000 hectares.
The other "primeval" forests are relatively small patches, they're not proper self-sustaining forests, Białowieża is the only one remaining. The biggest "primeval" forest in France is 25sqkm, Pyhä-Häkki in Finland is 13, Perućica is 14, Puzzlewood could be called pocketwood.
Białowieża is 3000 sq km, the World Heritage Site portion is 1400sq km.