Bouncing Zabaglione Bros.
Legend
The surface area isn't increased much by the slight corrugation. Yes, it will radiate somewhat, but it's not a tremendous departure from previous designs.
The difference is that it is not merely a plastic shroud that forces the air through the fins and out the exhaust as with most of these shrouded designs. It is actually another radiating surface that will take heat away from the heatsink and the hot air going through it. It replaces a non-radiating surface with a radiating one.
Looking at how the metal side parts of even a 48xx/58xx series heatsink can get too hot to touch, I think this will get very hot indeed, especially with 250w + to dissipate. That's what it's designed to do.