That claim seemingly stems from a misinterpretation of the paper’s discussion, since no mechanisms are proposed or mentioned there:

> The evolution of the vertebrate immune system occurred in geographically limited populations. Before machine-mediated transportation— trains, planes, boats and cars—individuals were likely to be exposed to the majority of pathogens in their local geography by the time of reproductive age. As T and B memory/stem cells can survive an individual’s lifetime[60,61], they should be sufficient to provide adaptive immune memory to all local pathogens. Thus, the generation of new T and B lymphocytes in later life was probably no longer advantageous, whereas the production of short-lived myeloid cells would remain important for acute innate responses, even in later life.