The journal's summary puts it as such:
We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable.Essentially, the richer a nation is the better able it is at building and maintaining a water infrastructure to serve all of its citizens than a poor nation can.
The BBC article provides a great visual that gives you an idea of the stresses associated with water development projects around the globe.
The top image shows water related stress threats in a "natural" state. One that excludes the influence of development projects. As you can tell, large swaths of the United States and most of Europe would be classed as highly stressed. However, the bottom pictures shows that management, such as irrigation and reservoir systems have minimized these threats in those regions.
Another region that shows noticeable difference between "Natural" and "Managed" is Africa. However, here the situation is reversed. Water resources are available, but the infrastructure simply isn't available in most cases to deliver that water to the people with the main barrier being cost. Coupled with the environmental costs associated with this sort of management, the journal authors make the argument against Africa and other developing economies from following the course of dams, reservoirs, and irrigation canals that are prevalent in the developed world.
Whether these regions follow the ideas of integrated water management to balance out the need to manage water usage against preventing harmful environmental impact is hard to say. Nations are still too willing to see dams and other monumental water projects as a symbol of political power to the people. Stripping away the grandiosity and pomp of concrete and steel and leaving behind the more complex reality of sustainability might still take a while to accomplish.