Wasn't neutronium practically synthesized in miniscule amounts in the Large Hydron Collider? Also I am not a quantum physicist, so I am not sure if any neutronium is currently safe to handle beyond a miniscule amount considering a sugar cube sized amount of neutronium is theoretically the weight of a large freight ship.
He said "physically" which is wrong because Neutronium. What he possibly meant was "practically" in which Osmium would be the only element you can practically fit in the box since it isn't possible to synthesize neutronium at that amount or handle that much safely.
Sounds like Nietzsche is a lot like me. We both snorted stuff.