Perhaps you've misread the quote? It and your counter affirm each other.

The quote suggests that there won't be labor shortages in markets that have adequate and adaptive immigration policies, because migration into a country with an aging population is how countries today maintain their labor pools.

That the global population will continue to grow into 2100 supports their assertion that all countries should be more immigration friendly so as to reap the bi-directional benefit of available labor moving into geographies that need workers.