I read the vox article, some of the things there are nonsense like with Belgium, an author saying that the Belgian royal family has the fortunes of King Leopold of the Congo fame (very unfortunate) which is not true because this current branch are not descendants of him and parts of the fortune went to his mistress whose toyboy gambled all of it and the Belgian government at the time of his death took a good portion of it when his estranged daughters were asking for it. The villas he built in the South of France La Leopolda and La Cèdres were built or purchased with the Congo bonds and monies but they were sold..
The bulk of King Leopold II's fortune was placed in the Royal Trust , which, despite not being privately owned by the Royal Family, is nonetheless at the disposal of the King and his successors in perpetuity.