There are multiple documented cases where a person caught COVID twice (where the two instances were genetically different, and therefore not a false positive caused by lingering dead virus).
The linked article only indicates that antibody production should continue for a long time, but that's only one factor in robust long-term immunity. The rate of antibody production and the set of proteins that the antibodies target are also very important.
It's good news to be sure, but it doesn't yet prove what you seem to want it to.