I think it’s simultaneously expelling heat and providing reaction mass to a ship that is massing a few kgs with a maneuver drive that neutralizes most of the mass.You're missing the real world constraint of inefficiency.
You're also making a canonically false assumption of 1 tonnes mass per Td; in looking at Striker, MT, TNE & T4 designs, mass is closer to 10 tonnes per Td. Knocks it up a factor of 10.
Note that the fuel rates given would be something astonishingly low recovery by book rates. 1 tonne of Hydrogen should be enough for multiple megawatt months.
Ty Beard and a couple others have done the math, and either the fuel is used mostly (around 98%) as working fluid rather than actual power, or the 250 MW is a recovery of under 10% of energy released... or both.