What the commentators don't understand is even if it was possible for billions of more humans to live on Earth due to a sustainable lifestyle there would first need to be a radical change in EVERYONES lifestyle. Now how likely do you think that is going to happen? If most of humanity hasn't realized their ways are destroying the planet are they suddenly going to change now or in the future? Is it even possible for many of us to go trotting off to our organic farming commune? I know I certainly haven't found the change yet.
The author is trying to say that overpopulation is a problem because of humanity's current overconsumption pattern and he's absolutely right. He's not trying to say that overpopulation in itself is an issue, but the ecological destruction that comes with having so many humans living on Earth is the real issue.
And for anyone who understands environmental issues and history will know that when an area has a large population of people living there it tends to go downhill. Todays desert in Africa and middle east is a warning monument to the danger of a population becoming more numerous than the resources of the area.