Mental illness is “associated” with worse covid19 outcomes, including death. Mortality was also higher in schizophrenia, schizotypal and delusional disorders compared to mood disorders. Bacterial, viral, and parasitic illnesses have been found in patients with schizophrenia and other mood disorders, including depression—but few (if any) studies have attempted treatment of mental illness by treating the underlying infections.

The statement covid19 is more severe in patients with mental illness is consistent with the statement mental illness is a medical illness. Patients with underlying medical illnesses, i.e. underlying chronic infections, are more vulnerable to severe outcomes from a severe acute infection.

https://www.medscape.com/viewarticle/948571