It is a disease as in “your metabolism is slow” so you need to cut food even more. It is a disease as “you have problem with controlling your impulses and therefore crave food”
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.