In the paper they are actually producing both biochar and biocoal. The biochar is the solid fraction produced during pyrolysis. The bio-oil (volatile hydrocarbons) are then further distilled to produce biocoal. By tweaking temperature and process parameters they can likely optimize for production of one or the other.
It's carbon negative to use it as a soil additive, but you don't get to burn the bio-coal for energy, so it's slightly less profitable.
I guess you could do a bit of both and be exactly carbon neutral.