I've never seen that before but I think the design makes sense. I've seen a similar thing done to bicycle saddles for the racers and that works for those organs (the crotch area). I suppose this would work for the tail end of the spine, but honestly, I've never had an issue with pain at the end of the spine, and considering that not many people are demanding regular chairs have this modification would lead me to believe there's probably a very small population that needs this.
They'd probably get more takers if they didn't charge so much just for the seat top. Their price for just that is more than an entire throne from somebody else.