Below is the entire paragraph. The first part of the paragraph summarized their research that declining populations do not decline in wealth or productivity; instead their Figure 8 shows increases in Wealth, Research & Development, Human Capital etc. The second part of the paragraph is obtuse and ambiguous wording where they advocate for increased immigration and as far as I could tell had no basis on their actual research.

      Our results provide a strong evidence-based counter to the politically motivated claim that declining/slow-growing and ageing populations in any way compromise national economic performance, income distribution, productivity, political stability, well-being, or health of its citizens. In fact, such populations generally have the highest socio-economic performance indicators in the world. This result supports the mounting evidence that smaller populations are in fact beneficial to most of society37, in sharp contrast to the unsubstantiated political rhetoric of ‘baby busts’ and an ensuing economic Armageddon1-5. This is because investing in the health, training, and education of workers — especially older, more experienced workers — increases human capital, making the workforce more productive29. Neither is there a basis for an expected penury of working-age people for countries experiencing low population growth or even decline. Labour shortages do not arise because of a lack of suitable workers, they occur instead because of inadequate immigration policies that limit or deny the movement of capable, working-age people from elsewhere to fill local demand70. Indeed, none of the existing credible population projections predicts a decline in the global population30,37,71-73.