I'm surprised they didn't ask/answer the obvious one: "even diamond?"
So I googled around, and it seems the melting/burning point of diamond at surface pressure is 3820 Kelvin, or 3548 Celcius, so he might be right.
That's a cool lens! I want one for my Canon 5D2.
Dangerous though. It's hard to believe, emotionally, that this point in space will burn off your finger if you put it there.
It seems it's a mirror-lens. I can't really tell from where the sunlight comes into it.