I was kinda wondering if the experiment used dogs of the same breed, identical age, similar life experiences, and at the same stage of hunger.

Article in Current Biology (pdf) :

The subjects were 24 dogs (50% male; estimated age range: 9-108
months) at two UK animal re-homing centres.

The experiment in fact concerned behaviour in circumstances of separation from usual human companions (no evidence offered on the dogs' affect wrt those humans). It proved, the authors say, that "pessimism" is caused by underlying mood.

Science is even more wonderful than I thought. Journalism too.