I'd say that the tubular CF frame probably has enough internal buoyancy to keep the frame, rig, crew afloat while not sailing. In that sense; it is not zero buoyancy, if it were it would go to the bottom of whatever lake or bay they were sailing in when it stopped. Seems like a small planing hull would be a better choice since it would reduce friction while sailing in non-hydroplaning conditions. (A design like the Moth is more practical IMHO).
I have to agree with the "bottom of lake" scenario. In the article they claimed "zero buoyancy", but I guess they stretch the truth a little.