Fusion on earth (that people talk about) is about generating electricity. Fusion on spacecraft is about shooting out exhaust gas as quickly as possible and storing energy at high density. The rocket equation relates the exhaust rate to the propelent mass requirements, which determines the feasibility of the spacecraft. It's a completely different optimization problem, and one that has historically favored nuclear energy sources in general.
To clarify further, from TFA: "While early versions of the Helicity Drive wouldn’t exceed the Lawson criterion (more energy out than in) and would require external (presumably solar) power, the energy added to the exhaust from fusion would increase the system’s exhaust velocity and ISP (giving somewhere in the neighborhood of 10,000 s isp, vs a maximum of ~400 for chemical rockets)"
So it is not able to generate more energy than it takes to operate, just like the fusion devices we can already create on earth, but can create really fast exhaust which makes the rocket more efficient.
