Note the bold highlighting of the word "unknown" - which is preceded by the word "not" for dramatic, sensational and confusing effect. In other words, the alloy is known.
Real question is - what IS the alloy? It's certainly not out of the question that a "hollow" ball of this size and weight could fall intact from a satellite of some sort with a decaying orbit. Titanium or some sort of titanium alloy jump to mind, since titanium has an extremely high strength to weight ratio (i.e., it's a strong light metal).
Also heat resistant.