Not so much complicated, as false. That myth has been dispelled a long time ago.
https://intermountainhealthcare.org/blogs/topics/covid-19/2020/11/debunking-the-myth-of-non-vaccine-herd-immunity-in-covid-19/
The article you referenced addresses the issue from the standpoint of natural herd immunity. It doesn't address adding vaccinations to hardly any degree. At the time it was published, in Nov 2020, vaccines were not even out yet and they looked at it from a non-vaccination standpoint.
In America, there have already been over 60 million vaccinations or roughly 18% of the general population, higher if you eliminate children in the population who don;t appear to spread the disease or get infected to a large degree. Some have estimated we may have 33% infected already naturally, so we're at 50% when you add the vaccinations to date.
The fact is the spread is down to about half of what it was just a couple of months ago. We're running out of people to get infected as well. Finally, herd immunity is a well-established medical understanding. The writers of that article need to update their predictions. They're wrong.
