Those authors are not doctors nor am I, but herd immunity will very likely if we follow Dr. Redfield’s advice (CDC DIRECTOR) and universally mask, which reduces viral load both in and out, and which tends to reduce deaths and the severity of the disease.
There is no such thing as herd immunity as no immuity is provided. What is call “herd immunity” is actually a simple lack of exposure. Once an infected person from outside the herd enters into the population, those that never developed immunity can be infected.
They don’t claim to be doctors.