This is misleading, it conflates the presence of antibodies with immunity. There is no reason to believe immunity is not durable; a strong immune response to related coronaviruses (e.g. SARS-CoV-1) lasts decades. Antibodies are only generated in response to infections, and disappear after the infection has passed at varying rates based on a number of factors.

People have no antibodies circulating for most things they are immune to. It is expensive to produce antibodies unnecessarily.