There's have been several studies, well researched and cited, where people who claim to be "diet resistant" are given metabolic markers "double labeled water" that will accurately show caloric intake.

For example:

https://www.nejm.org/doi/full/10.1056/NEJM199212313272701

and

https://journals.physiology.org/doi/full/10.1152/ajpendo.200...

In the NEJM article they note that every single person who claimed to be "diet resistant" was lying about food intake.

> The main finding of this study is that failure to lose weight despite a self-reported low caloric intake can be explained by substantial misreporting of food intake and physical activity. The underreporting of food intake by the subjects in group 1 even occurred 24 hours after a test meal eaten under standardized conditions. In contrast, values for total energy expenditure, resting metabolic rate, thermic effect of food, and thermic response to exercise were comparable with those of obese subjects in group 2 who did not report a history of diet resistance.

and

> In addition to their greater degree of misreporting, the subjects in group 1 used thyroid medication more often, had a stronger belief that their obesity was caused by genetic and metabolic factors and not by overeating, and reported less hunger and disinhibition and more cognitive restraint than did the subjects in group 2. Subjects presenting for weight-control therapy who had these findings in association with a history of self-reported diet resistance would clearly convey the impression that a low metabolic rate caused their obesity.

Calories-in/Calories-out is true for everyone, and everyone can lose weight by putting down his fork.