It isn't a Sigma deal. It's a LensRental deal. They paid $26,000 for the lens, rented it once for about $1000 and are selling it for $15,000. They lost at least $10,000 on the lens. Incidentally, they also sell used Canon equipment.
I'm guessing Roger C. and other LensRentals employees got to play with it a LOT. Good from them. Perhaps one of the perks of working there?