It is normal for antibodies to decline after an immune response, it doesn't mean the immunity is gone, the immune system remembers how to produce them when need

It is normal for antibodies to decline after an immune response, it doesn't mean the immunity is gone, the immune system remembers how to produce them when needed. The paper you link actually says that the memory B-cell populations appear to be maintained after the waning of the antibodies. B-cells are the cells that remembers and creates antibodies in response to an antigen (like the spike protein on the surface of the virus).