Without getting into the discussion of whether or not your problem is simply equivocation because you are interpreting "unable" in a different sense than those using it (or, e.g., the medical community) would in this context, "eating less" in the context of this discussion was eating less volume of food with the same calorie density, and the post to which you responded was pointing out that that is, in practice, not as successful for some people as eating similar volumes of food with lower calorie content.
Both of these methods are equally "eating less" in a caloric balance sense. The set of processes that affect processes related to wait loss (including both processes that control how much of that energy content is actually made available, and processes that might be viewed as compliance-related in terms of satiety triggers on the input side and processes that make people prone or resistant to activity on the output side) have substantial, well-known, biological variation between individuals, which have significant linkages to propensity for obesity.
Treating the issue one of simply calories in vs. calories out with success outcomes differentiated only by moral features like willpower is a stupid statement which is proven false by a vast body of research on metabolism, obesity, etc.
If you look at statistics from Europe at the end of WW2, or from the late 1930s in the US, how many obese people were there? Pretty much zero, because food was scarce.