Re: low antibody response and the possibility of reinfection.
1. The study is subject to survival sample bias.
There could also be a confounding factor related to disease duration, age and time for a full adaptive humoral response(though even mild disease duration seems enough?)

2. As with the in vitro study conducted in that same paper, there is indication that the antibodies are, in practice, neutralizing, and recovered plasma therapy continues to show promising (but not yet proven) results.

3. If true short-term reinfection were occurring to any significant extent, countries with broad contact-tracing and testing programs, such as South Korea, would have found people who had been infected by them by now. To the best of my knowledge, this hasn't occurred.

4. If antibody production is indeed deleterious, perhaps another factor to consider, beyond ADE, is involvement in thromboembolism.
@mlipsitch @nataliexdean @MedCrisis @methodsmanmd