I would call it simplification not pseudoscience. But older people having weaker immune systems in general is a thing. You’re picking at the individual pieces of why but they matter less to me than the holistic reality. Not all kids are safe. The big majority of old people were never doomed. But on planet sized problems generalizations are critical in communicating with even doctors. We spent decades telling people that only barriers work for HIV and that lambskin has tiny holes that virus can get through but now that a tee shirt will be the difference between life and death.