This is a good point. While "calories in != calories out" within a broad range of caloric inputs, because humans have some built-in ability to absorb fluctuations in food intake centered around each person's metabolic setpoints, you can definitely get some play at the extrema of the caloric intake/expenditure axis.

In the opposite direction from your example, if someone has a hard time gaining weight, they may find that eating 3000 cal/day has no effect but eating a carefully measured 6000 cal/day definitely moves the needle upward.

The problem in general is that maintaining a caloric deficit of 2000 cal/day for weeks is going to be impossible to achieve for most people, and likewise maintaining a caloric excess of thousands of calories per day is a full-time job (ask any bodybuilder).