Pretty simple - its efficacy for malaria, arthritis and lupus have no bearing on whether it might help with a virus (after all, none of the above are viral diseases, nor have anything else obviously in common with COVID-19). The only way to know for sure is to test - properly - but even if it turns out to help, it was still a long shot. While the evidence so far is weak, hospitals are stocking up just in case - so if it turns out it does work, they’re ready to go ahead with it.
Doesn’t change the fact that it’s a dangerous drug with nasty side effects ranging from merely uncomfortable to lethal in some individuals (heart arrhythmia being perhaps the nastiest).
