If you go to the source article, this is stated as: >our SBMT cell shows peak charge and discharge power densities of 1,322 W/Lcell and 306.1 W/Lcell, respecti

If you go to the source article, this is stated as: >our SBMT cell shows peak charge and discharge power densities of 1,322 W/Lcell and 306.1 W/Lcell, respectively, compared with average charge and discharge power densities of <60 W/Lcell and 45 W/Lcell, respectively, of conventional planar flow battery cells. So, it's not per volume of electrolyte, but per volume of the entire cell (housing included).