No an epidemiologist, but I think the theory is you only need herd immunity in the local area, if travel is also restricted, for the infection to burn itself out.

We really have no idea what fraction of the population caught it, since so many show minimal symptoms.

If it were not a significant fraction in infected areas I’d expect it to flare up again quickly. Quarantine simply cannot be maintained indefinitely, nor is it going to stop all spread.

Do we know how many people are already susceptible to it? It’s plausible that some percentage of people are immune to it already — maybe they recently caught a similar coronavirus or some other natural resistance to it.