is is plain rock that has grown SW
bacteria and coralline algae...
So long as the surface is porus (the more porus the better) it will be beneficial.
I have two hand made pieces of compressed limestone that are starting to take one aspects of liverock (turning red/purple/brown instead of the bright sandy color). Its superporus and also strings beneficial calcium into the tank over an exceedingly long period of time.
Looks like the guy is making molds and dropping regular concrete (or atleast saltwater non-dissolvable) into them.