You seem to be ignoring the minor fact that lower molar mass also means more moles, so it cancels out.

If you pump 1kg of water into the engine, that's 55.5 moles. If you pump 1kg of hydrogen into the engine, that's 500 moles.

So hydrogen produces 1/9th as much force per mole, but it also has 9 times as many moles per kg of fuel. The end result is that both produce the same total force when that kilogram is expelled from the engine.

Or at least, they would if they were both expelled at the same speed. Since hydrogen actually comes out twice as fast, it produces 1/4.5th as much force per mole, while still having 9 times as many moles, and hence produces twice as much total force.