In the mental health field in particular, it's not unusual to list various not-a-diagnosis problems, stress factors, life circumstances type things, in the diagnosis list (see also: social determinants of health). E.g., a lot of my patients are homeless, and I also work with a lot of forensic patients, so their diagnosis lists often includes "homeless" or "legal problem". Which, obviously, aren't actual diagnoses, but it's often the best way to communicate to other members of the care team or future caregivers what the major factors are influencing a patient's health. For many people, "low income" is a major source of stress which drives or exacerbates their mental health problems, so it does make sense to include if the therapist thinks it's a factor.

Now, why it's the only item listed is another question entirely. It could be a quirk of whatever system they use for patients to view their records; with the electronic medical record system my employer uses, on some screens it only shows the first item in the diagnosis list, so if I put in "homeless" first then that's all I would see on some pages. The system isn't smart enough to know what's an actual diagnosis or not, so it relies on humans to put in the data correctly.