It's a fine idea but there's problems with this. The turbine must be small so the efficiency won't be very good. It needs either a big radiator and/or big expansion tank to re-condense the water. And it'll be hugely more expensive and heavy because you're replacing the alternator and belt with a turbine, generator, exhaust heat exchanger, radiator/tank, plumbing, and possibly a water pump. They conveniently only show the exhaust heat exchanger in the picture.
At least with their peltier junction idea there's no moving parts.