That's my complaint, it doesn't have much greater energy density, or more precisely it's not usefully translated to momentum. The speed target in the article is

That's my complaint, it doesn't have much greater energy density, or more precisely it's not usefully translated to momentum. The speed target in the article is extremely low ("62,600 mph") -- only a small factor higher then what chemical rockets achieve, and comparable (as I mentioned) to nuclear thermal rockets. (It looks like they edited their article to mention NTRs.) They have extremely energetic fuel, but they don't get any meaningful specific impulse out of it. Maybe they don't easily extract momentum from high-energy radiation. Or maybe they lose too much mass to radiation damage or vaporization ("ablative shield") -- their mass consumption could be dominated by shielding, not fuel. Their ultimate goal is to develop a nuclear fusion propulsion system by 2030 that can spirit spacecraft from Earth to Mars in around three months—about twice as fast as researchers think they could go with a nuclear fission engine, another scheme that is being investigated but has not yet been built.