That's these victims' best case scenario. As more people fall ill, it appears the experimental treatment strips away the natural immunity they would have built up from being exposed to COVID-19 since 2019.
There are an increasing number of cases of individuals who beat COVID-19, got the experimental treatment (who knows why?), and are now fighting a worse case of COVID-19 than they faced initially.
Contrast that with those who built up natural immunity against other coronaviruses such as SARS 2003. They've been shown to have antibodies against SARS COVID-19.
You can't really have it both ways. Immunity is immunity. Antibodies against COVID are the same as antibodies obtained through vaccination.
No, they've been shown to have some cross-reactive T-cell lymphocytes in a laboratory setting outside the body. It's a promising sign there may be some level of resistance, but without challenge studies (which are unethical with a deadly disease) or epidemiological studies (difficult considering there are only about 7,000 known SARS 2003 survivors), there's no evidence so far that immunity exists. And it would not be possible to have antibodies to a different virus as they're specific. If you want less specific (increased cross-reactive potential) you have to look to the antigen receptor sites on T-cells.