Nick,
It's not differentially hardened, it's D-2 at a 58 Rockwell. Beadblasted overall to begin with (not the handle), then the grind was hand-polished to satin, then the hamon "effect" was apparently polished on top of the handrub. At least that's what I gather, from what I can see and from what I've read about the piece. I've not been able to exactly nail down from anything in writing how the hamon came about, so that part of my explanation is purely from my own observation.
But, of course, it really doesn't matter, since this piece seems obviously meant to be an art piece, one that perhaps exists to represent the form moreso than the function. It's the art of it that caused it to be acquired rather than any functionality - but it is indeed plenty sharp.
But it should be noted that I have a great disdain for any type of hamon, true or otherwise. Still, the piece appealed to me enough to acquire.
Bob