I’m not sure what that proves. There are objects in the universe that are many orders of magnitude more dense than osmium, which is still mostly empty space. A spoonful of neutron star “stuff” would have roughly the mass of the earth.
It was an intellectual exercise. I wasn’t out to prove anything. I just wanted to know how big a cube, that had all the matter of the universe, would be if it was as dense as the most dense element that we know of.