The wiki article doesn't really have any reliable sources/citations. It only has one citation. There is a lot wrong with what is presented, especially in the concepts described by the two figures.
One big problem is that fission fragments in solids have a range of travel on the order of 4 to 7 microns, the heavier particles traveling in the lower range, and the lighter nuclei traveling in the upper range.
Alternative concepts have called for gaseous core reactors, but those have low fission density - fissions per unit volume.
In propulsion, high Isp systems usually have low mass flow rates and low thrusts, and require a lot of power. The trade off is low mass of stored propellant at the expense of thrust. The ultimate goal in propulsion is to maximize specific power or power density, while minimize stored propellant (lower mass to accelerate), but subject to the constraints imposed by the mechanical (physical) limits of materials (tensile strength, creep, fatigue resistance, fracture toughness, melting point) with which we much construct the propulsion system.