Quite interesting ...

The manufacturing process is apparently big secret, maybe rtm but you would still need some sandpaper to get rid of the parting lines.
They claim:
-No sandpaper.
-No paint brushes.
-No tape.
-No solvents except water.

Anyway the materials are quite cutting edge.

-ORGANIC BASALT HONEYCOMB TECHNOLOGY

  •                         Instead of toxic industry-standard polyester or old-fashioned  epoxy, we use a new chemistry that has higher elongation, extreme ding  resistance, and responsive rebound.
    

http://www.libtechwaterboarding.com/waterboards/

so, probably a thermoplastic (maybe expanding) core to fill the basalt honeycomb. glassfibres and alutex fibres, thermosetting bio based resin? or a thermoplastic variant. Expanding core could provide pressure in a closed mould.

Interesting!

Bottom texillium?