The article didn't seem to have much more info than TFS so I'm guessing here, but in general, you may not need to generate net power for fusion to be useful for rocket thrust.
There's plenty of solar power available in space (at least near the inner planets), and as the Farnsworth fusor showed decades ago, it's not hard to generate fusion reactions if you don't expect positive energy output. For some space missions, propellant mass is very important, and getting the highest velocity exhaust is the goal. Using solar energy to induce fusion reactions could be one way to do this. Of course, like most ion drives, this would be very low thrust over long time spans.