Source?

The combination of our global population and our resource consumptions per capita have devastated ecologies the world over and led to the deaths of innumerable species and biospheres.

You basically have the choice, then - massively reduce the resources consumed per-capita such that having billions of people doesn't negatively impact the planet or displace the last remnants of nature unsullied by human encroachment, or reduce the population so that the people you do have can consume resources without causing a mass extinction event.

When population reduction is as simple as "stop having kids" that is by far the least harmful mechanism to currently living humans to mitigate the ongoing disaster we are causing the planet. Trying to tell the current first world or developing countries to stop building roads, driving cars, building houses, using convenience products like plastics, stop building technologies that require scarce rare earth minerals that require extraction, stop using electric power in its myriad forms, etc is way more disruptive and harmful to those living today.