That is trivially true at the extremes of energy input. If you input an infinite amount of energy you will not get an infinite amount out.
But that’s not what we’re talking about. This is a physical process which is known to be exothermic for the energy ranges we care about.
As another example, raising the temperature of a flammable material 1 degree from room temperature will probably not light. Ditto with 2 degrees. But eventually, if you raise the temperature high enough, you’ll get more energy out than you put in. That’s the type of process we’re talking about now.
