People have literally died of that mistake

Of that mistake? Links, please, to cases where people actually bothered to control their electrolytes, vitamins, minerals, and other essentials -- they just denied themselves calories and died from starvation still weighting a few hundred pounds.

people who who have a lot of difficulty with losing fat shouldn't be assumed to be lying

"Have a lot of difficulty with" and "biochemically cannot" are very different things. A lot of people have difficulty with losing fat -- this does not imply that they cannot do it, only that they are unwilling or incapable of paying the price to do it.

may be losing more muscle mass than is good for them

Alternatively they may be accumulating more fat than is good for them.

As a practical matter I am not a big fan of using weight as the target variable. I much prefer either the body fat % or getting naked in front of a full-length mirror.