I'm chalking this to the author not understanding what they were writing. Because if you read the article they flip back and forth between stating the growth rate will peak at that population marker, followed by that the population itself will peaked then. And those are two totally seperate peaks, honestly don't think they could predict the highest possible population that would be harder to determine. NOW, looking through some other info it does appear it is the population that peaks around 8.6 billion.

You need to restudy calculus. Because that's exactly what my point is derived from - see what I did there?? The growth peak will not be the same time as the growth rate peak ( a secondary difference). The growth rate will peak, and THEN when the growth rate hits 1 to 1 the population will begin to decline.

What your stating would be true if humans had kids on a 1:1 ratio, but if the population growth rate peaks at 4 children per 2 adults than there will be a period of time when the growth rate declines from 4 children per to 2 children per and that will still see an increase in population, even post peak growth rate.