He's certainly done a good job with that dendro. I heard that some Japanese reef-keepers are continually restocking too, but that wouldn't be the case here
I'm not sure how he does it either. I think it is more about providing the Dendro's with what they eat rather than having high nutrients. Dendronepthia eat Phytoplankton from my understanding. Maybe he doses Phyto heavily. I have also heard that the Japanese get Dendro's for the color and then throw them away when they die.
This corals are not dendro,that particulary coral from the picture is nephthea chabrol and is photosysntetic.