https://document.environnement.brussels/opac_css/doc_num.php?explnum_id=4115 say that the "park" is 6 hectares 22 ares ( parc régional) 

In OSM https://www.openstreetmap.org/way/284512062 is 6.614 ha so probably the outline is not 100 % correct but certainly not the all area (in osm +- 40 ha ; 50 hectares for the region (you need to include the park in the count so missing 4 ha) to have the all "scheutbos"