Originally Posted by Valsdad
From the actual published article:

https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(20)30764-7/fulltext


From the article ...
"The mechanisms that could account for a more severe secondary infection can only be speculated. First, a very high dose of virus might have led to the second instance of infection and induced more severe disease.Second, it is possible that reinfection was caused by a version of the virus that was more virulent, or more virulent in this patient's context. Third, a mechanism of antibody-dependent enhancement might be the cause, a means by which specific Fc-bearing immune cells become infected with virus by binding to specific antibodies. "

Also ...
"A major limitation of our case study is that we were unable to undertake any assessment of the immune response to the first episode of SARS-CoV-2 infection."

So they don't know if the person developed antibodies from the first case.