Nobody has a metabolism that is defying thermodynamics and creating mass that you didn't intake as food.
If their temperature isn't 98.6 degrees Fahrenheit or very close to it, they are metabolically compromised (note, of course, that people will practically never have a resting body temperature of greater than 98.6)
I had a full dexa scan and calorimetric exam done and my body was burning approximately 300 calories a day under what it was supposed to. My resting body temperature was 97.3. If I ate what my TDEE should have been I would have been putting on half a pound a week or more.
Eating more and exercising to rebuild muscle lost to over dieting has corrected that issue.
But saying that it doesn't exist is completely wrong. The math is still "calories in, calories out" but you can't assume that "calories out" is static and inflexible. There are many variables at play in the human body.
Have you ever done the math?
200 calories can mean the difference of two people eating the exact same calories every day where one maintains the same weight, and the other gains 2 lbs a month, or 24 lbs a year...nearly 100 lbs in just 4 years!
You are proving the point that variance in metabolism can be a HUGE factor.
(I have done the math.)
But my point stands that a few percentage points difference in metabolic rate makes a big difference. And likely could lead to one person becoming overweight while another person with identical habits maintains a healthy weight.
If you compare two neighbors, you're right.