Psychological diseases are not better or worse from psychical/metabolic ones. They are real, and for some of them we have or we develop medicine.
Nobody is claiming that obesity can’t be a result of a disease, but under the hood it always ends up as: calories surplus is stored as fat.
So, slow metabolism is not a disease, it's not a genetic disorder, it is simply a result of the fact that someone is spending too much time on the couch.
I think this gets lost a lot when people talk about "slow metabolism", they turn it into this thing they're just helpless to influence, like they're just cursed with a slow metabolism and that's that. It's not like that at all, which is why I don't like the term. It just hides the reality of the situation.