The article doesn’t talk about population. It discusses many other things but primarily focusses on lack of capital formation and over reliance on debt in its place.
But population is critical. It is the demand side of supply and demand. People need food, shelter, etc. and more people = more demand = more GDP. Less people = less demand = less GDP. Yet our current economic model can't allow declining population and declining GDP, because it is debt based. Our money is debt based money. How does a debt based money work in a shrinking economy? It just makes the debt more unpayable.
And I believe the most important thing is that once the collapse really gets started, it will be self fueling, because the population decline will cause a never ending depression, which will cause adults to not want to have children they can't afford and can't guarantee they will have a life even equal to their own, which lowers the population, and causes greater economic contraction.