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 It is also a preprint and appears it has not yet been peer reviewed.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.