It is not clear the patient’s were matched well, i.e. the the ones that got HCQ were probably sicker. If I did a study of COVID19 patients for outcomes between those who were on a ventilator and those that were not, certainly those on the ventilators would have much higher death rates.
Yes, it is VERY possible that HCQ was given only to the sicker patients. When a study says “retrospective” (which it does) that means the above is possible.
You can only have confidence in randomized double-blind studies, and this one doesn’t qualify.