> All the info that's out there says that immunity to C19 is temporary

This isn't quite true. What is true is that there is no information out there to conclusively support long term retention of antibodies, in large part because we've only got 6 months of data.

As to the latter point, I wonder about the trial design. If they didn't give all participants a Covid antibody test before starting, it is very likely that several participants were previously infected. Smaller sample size though.