Background:

Lupus erythematosis is an autoimmune disease. Chloroquine or hydroxychloroquine are used to treat lupus erythematosis by acting as an immunosuppressive. In around Dec 2010/Jan 2020, Chinese doctors discovered that no patients with lupus erythematosis became ill with COVID-19 and hypothesized that the chloroquine or hydroxychloroquine that they were taking might be the reason. 

What is the mistaken reasoning here?

1. Patients with lupus erythematosis who are taking immunosuppressives are medically vulnerable. They know this. Hence, under normal conditions–to say nothing of under conditions of a pandemic (at the time epidemic)–they are the most likely to take precautions such as social distancing, hand-washing, and mask-wearing. This is probably a case of false cause.