They actually studied the skeleton as well.

> The body was placed in a large pottery vessel inside a rock-cut tomb (Fig. 1b and Extended Data Fig. 1). This treatment would have ordinarily been reserved for individuals of a higher social class relative to others at the site

But,

> This and various activity-induced musculoskeletal indicators of stress revealed that he experienced an extended period of physical labour, seemingly in contrast to his high-status tomb burial.

> In this case, although circumstantial, they are not inconsistent with those of a potter, as depicted in ancient Egyptian imagery.

Checking the corpses of nobility would be a bad idea because they are shipped around for diplomatic reasons. I guess a potter moves around less (though, as a skilled worker, probably moved around a bit?).