To not bury the lead: His team studied data the government made available from military missile tracking systems, which conveniently also false-positives on asteroids. There was one in 2014 which slammed into the Pacific Ocean at a speed at least twice that of the speed stars around us move relative to the sun, which makes it likely to be extra-solar. Based on that speed & how much burned upon entry, they concluded i…
But the "tougher than iron" part? Based just on how much material was lost during atmospheric entry, as reported by sensors not really designed for that purpose? That's a tough pill to swallow. It would be really interesting to hear more about how much this asteroid differed from others measured by the same equipment.