Scientists have found that people who had SARS are immune to COVID. Their T cells fight it off. There was another variant of a coronavirus that went around and people who got that seem to be immune to. Some scientists testing on older blood samples found this. The older coronavirus didn't cause major issues in society and went unnoticed.
I share this to say that there is reason to believe that we will be able to fight this off, even with mutations. It's not baseless hope.
That study has N=24 and only studies T-cell response, which is a long way from being "immune" to infection. It is also a preprint and appears it has not yet been peer reviewed.
So, basically, some immune response is not the same as immunity against COVID-19, which is what your OP (over-)claimed.
But it really doesn't tell us anything we didn't know. SARS-COV-1 and SARS-COV-2 are related and there is some cross-activation of immune response. It is not an epidemiological study or anything.
I share this to say that there is reason to believe that we will be able to fight this off, even with mutations. It's not baseless hope.