Considering the abrupt and vague comment you made, it was incredibly easy to interpret it as an assertion that metabolism itself was, to you, a myth that "isn't backed scientifically". You made no further clarification, and despite your continued refutation, there is scientific evidence of different basal rates of metabolism. Granted, this difference is by and large only marginal. The point, though, is that this has much more to do with the cat than what you're letting on. Despite all good intentions and efforts from the owner, some things simply cannot be helped because the problem is internal.

Larger organisms have "faster" metabolisms. They are able to burn more calories per day because of this. Smaller organisms have "slower" metabolisms, and do not burn as much. This is also highly dependent on levels of activity; more active organisms burn more, whereas less active organisms burn much less. If you read on and saw the user's later comments, we can see that a number of different diets have been administered; the calorie control you mentioned prior. It didn't work. The cat is also older, and is likely much less active than it used to be. The result is a fatter cat that doesn't seem to show any progress, despite the quantity of food it is fed. Therefore, it is not the fault of the owner. He's looked into solutions and has tried many. Things can't be helped if the cat is simply not active. There is no reason to continue to blame them, and it needs to stop.