But our behavior influences the value of R0. I suspect that most of us have been thinking about "herd immunity" only under the conditions that we lived life, say, last year. Under those conditions, estimates are that something like 70% of people had to have immunity to Covid to achieve R0 < 1. (I should note that we don't even know yet if people who recover from the disease gain immunity, or how long it lasts if they do, let alone whether a vaccine will be developed.)
However, under a different set of behaviors, the transmission rate is different. So mask-wearing, hand-washing, etc., are not just buying us time, they are altering the dynamics of the spread of the disease, that is, they are altering the threshold when "herd immunity" is achieved. Of course, the "herd" is a different herd than we are used to; our behaviors would be different. But we as a society could choose to be part of a herd where the disease doesn't spread as rapidly (at some social costs), and the herd immunity threshold is lower. Or we could choose to be part of a herd where the disease spreads rapidly, the threshold of herd immunity is much higher, and many more people are sickened or die.