BuGGeH
Re:Comparing SARS-CoV-2 natural immunity to vaccine-induced immunity [...
on Wednesday August 25, 2021 @09:03PM (#28309306)
From your same paper.
>These findings suggest that among persons with previous SARS-CoV-2 infection, full vaccination provides additional protection against reinfection
What "2.34 times" is referencing is two groups had COVID-19 but one of them got the vaccine as well. In which case the COVID-19 + Vaccine group has less chance of reinfection compared to those who only had COVID-19.
The original article does mention that COVID-19 + infection does provide more protection as well I believe.