What this shows is that a single dose administered to a previously infected person provides greater antibody production than a non infected person's recieving two doses. The study also says that they are unsure if those higher levels affect a patient's ability to get sick or transmit the virus.
What I would be looking for is a study showing that prior infection has x% effectiveness as measured against a vaccine on a similar timeline.
[1]: https://www.nejm.org/doi/full/10.1056/NEJMc2103825?query=TOC