Except it's not. The body is not a closed system. The law of conservation of mass & energy just means you have to balance your chemical equations. It doesn't necessarily mean eating more will make you fat. (Maybe it does, maybe it doesn't ... it's just not necessarily the case.)
Appeal to intuition: you can eat 10k calories of straight lard, and you will crap most of it out. You can do other things to "burn off" that excess energy, too, like build muscle, raise your body temperature, etc.
To be clear: I don't know to what extent (if any) the body does any of the above things. My point is that the "calories in vs calories out" meme is so inaccurate that it borders on dead-wrong.
In any case where someone is mentioning CICO seriously, it's meant in the sense of a regular human diet (not necessarily balanced and healthy). Making a point from a fallacious example is a waste of time.
The quickest way to gain two pounds is drinking 32oz of water.