No medium it looks like, just water, just like a DWC system, without the hassles of dwc. The buckets are plumbed together, a pump pushes water through the root systems, and somehow he has added the rings to top feed the roots in addition. Another pump or just a t in the line.
RDWC are becoming commercially available, but the best systems still seem to be homemade as this one appears. Not a terribly difficult system, just getting everything to seal is typically the issue. The grommets to seal the pipes to the buckets are expensive. I am sure a little research and you will understand what he has done here.