[barumi]

My take is that this is a "correlation does not imply causality" case.

It was possible to positively identify a reinfection because the two infections were due to different strains. That was just the means of positively and objectively identify a reinfection. However, nothing was asserted regarding if the patient was immune to the first strain the first time around. In fact, it seems that the hallmark of reinfections is that they are far more critical than first infections. This also means that the first infections is far milder than the second one. Well, the thing about the immune system is that it is able to fight mild infections without developing immunity if they are too mild to trigger a full response from the immune system.