It sounds like the argument is considering what the output energy might be used for. If you want a fusion device to power your heating and lighting, then the "waste" heat and light from the device may actually be useful.
It's like replacing the radiators in your house with servers or bitcoin miners; you still pay for electricity, and it's still warming up your house, but you get compute cycles or bitcoins "for free". If the waste heat/light of a fusion device is of comparable efficiency as conventional heaters/lighting, then we're "breaking even"; if the device manages to perform any fusion, that liberated energy would be a "net positive".
Of course, a fusor doesn't seem like a particularly good idea for household heating or lighting, and that argument is completely inapplicable to things like power plants.
It's like replacing the radiators in your house with servers or bitcoin miners; you still pay for electricity, and it's still warming up your house, but you get compute cycles or bitcoins "for free". If the waste heat/light of a fusion device is of comparable efficiency as conventional heaters/lighting, then we're "breaking even"; if the device manages to perform any fusion, that liberated energy would be a "net positive".
Of course, a fusor doesn't seem like a particularly good idea for household heating or lighting, and that argument is completely inapplicable to things like power plants.